Merge branch 'release/0.18'

connector_api/connection.h

@@ -51,6 +51,10 @@ namespace sqlpp
 		class connection: public sqlpp::connection // this inheritance helps with ADL for dynamic_select, for instance
 		{
 		public:
+			using _traits = ::sqlpp::make_traits<::sqlpp::no_value_t, 
+								::sqlpp::tag::enforce_null_result_treatment // If that is what you really want, leave it out otherwise
+							>;
+
 			using _prepared_statement_t = << handle to a prepared statement of the database >>;
 			using _serializer_context_t = << This context is used to serialize a statement >>
 			using _interpreter_context_t = << This context is used interpret a statement >>;

include/sqlpp11/alias.h

@@ -35,7 +35,7 @@ namespace sqlpp
 	template<typename Expression, typename AliasProvider>
 		struct expression_alias_t
 		{
-			using _traits = make_traits<value_type_of<Expression>, tag::named_expression, tag::alias>;
+			using _traits = make_traits<value_type_of<Expression>, tag::is_named_expression, tag::is_alias>;
 			using _recursive_traits = make_recursive_traits<Expression>;
 
 			static_assert(is_expression_t<Expression>::value, "invalid argument for an expression alias");

include/sqlpp11/any.h

@@ -35,7 +35,7 @@ namespace sqlpp
 	template<typename Select>
 		struct any_t
 		{
-			using _traits = make_traits<value_type_of<Select>, ::sqlpp::tag::multi_expression>;
+			using _traits = make_traits<value_type_of<Select>, ::sqlpp::tag::is_multi_expression>;
 			using _recursive_traits = make_recursive_traits<Select>;
 
 			struct _name_t

include/sqlpp11/assignment.h

@@ -30,48 +30,25 @@
 #include <sqlpp11/default_value.h>
 #include <sqlpp11/null.h>
 #include <sqlpp11/tvin.h>
+#include <sqlpp11/rhs_is_null.h>
 #include <sqlpp11/serialize.h>
 #include <sqlpp11/serializer.h>
 #include <sqlpp11/simple_column.h>
 
 namespace sqlpp
 {
-	template<typename T, typename Enable = void>
-		struct is_trivial_t
-		{
-			static constexpr bool _(const T&)
-			{
-				return false;
-			}
-		};
-
-	template<typename T>
-		struct is_trivial_t<T, typename std::enable_if<std::is_member_function_pointer<decltype(&T::_is_trivial)>::value, void>::type>
-		{
-			static bool _(const T& t)
-			{
-				return t._is_trivial();
-			}
-		};
-
-	template<typename T>
-		bool is_trivial(const T& t)
-		{
-			return is_trivial_t<T>::_(t);
-		}
-
 	template<typename Lhs, typename Rhs>
 		struct assignment_t
 		{
-			using _traits = make_traits<no_value_t, ::sqlpp::tag::assignment>;
+			using _traits = make_traits<no_value_t, ::sqlpp::tag::is_assignment>;
 			using _recursive_traits = make_recursive_traits<Lhs, Rhs>;
 
-			using _column_t = Lhs;
+			using _lhs_t = Lhs;
-			using _value_t = Rhs;
+			using _rhs_t = allow_tvin_t<Rhs>;
 
-			static_assert(can_be_null_t<_column_t>::value ? true : not std::is_same<_value_t, null_t>::value, "column must not be null");
+			static_assert(can_be_null_t<_lhs_t>::value ? true : not (std::is_same<_rhs_t, null_t>::value or is_tvin_t<_rhs_t>::value), "column must not be null");
 
-			assignment_t(_column_t lhs, _value_t rhs):
+			assignment_t(_lhs_t lhs, _rhs_t rhs):
 				_lhs(lhs), 
 				_rhs(rhs)
 			{}
@@ -82,8 +59,8 @@ namespace sqlpp
 			assignment_t& operator=(assignment_t&&) = default;
 			~assignment_t() = default;
 
-			_column_t _lhs;
+			_lhs_t _lhs;
-			_value_t _rhs;
+			_rhs_t _rhs;
 		};
 
 	template<typename Context, typename Lhs, typename Rhs>
@@ -93,7 +70,7 @@ namespace sqlpp
 
 			static Context& _(const T& t, Context& context)
 			{
-				if ((trivial_value_is_null_t<typename T::_column_t>::value
+				if (((trivial_value_is_null_t<typename T::_lhs_t>::value or is_tvin_t<typename T::_rhs_t>::value)
 							and is_trivial(t._rhs))
 						or (std::is_same<Rhs, null_t>::value))
 				{
@@ -110,52 +87,6 @@ namespace sqlpp
 			}
 		};
 
-	template<typename Lhs, typename Rhs>
-		struct assignment_t<Lhs, tvin_t<Rhs>>
-		{
-			using _traits = make_traits<no_value_t, ::sqlpp::tag::assignment>;
-			using _recursive_traits = make_recursive_traits<Lhs, Rhs>;
-
-			using _column_t = Lhs;
-			using _value_t = tvin_t<Rhs>;
-
-			static_assert(can_be_null_t<_column_t>::value, "column cannot be null");
-
-			assignment_t(_column_t lhs, _value_t rhs):
-				_lhs(lhs), 
-				_rhs(rhs)
-			{}
-
-			assignment_t(const assignment_t&) = default;
-			assignment_t(assignment_t&&) = default;
-			assignment_t& operator=(const assignment_t&) = default;
-			assignment_t& operator=(assignment_t&&) = default;
-			~assignment_t() = default;
-
-			_column_t _lhs;
-			_value_t _rhs;
-		};
-
-	template<typename Context, typename Lhs, typename Rhs>
-		struct serializer_t<Context, assignment_t<Lhs, tvin_t<Rhs>>>
-		{
-			using T = assignment_t<Lhs, tvin_t<Rhs>>;
-
-			static Context& _(const T& t, Context& context)
-			{
-				serialize(simple_column(t._lhs), context);
-				if (t._rhs._value._is_trivial())
-				{
-					context << "=NULL";
-				}
-				else
-				{
-					context << "=";
-					serialize(t._rhs._value, context);
-				}
-				return context;
-			}
-		};
 }
 
 #endif

include/sqlpp11/avg.h

@@ -35,7 +35,7 @@ namespace sqlpp
 		struct avg_t: public floating_point::template expression_operators<avg_t<Flag, Expr>>,
 		public alias_operators<avg_t<Flag, Expr>>
 	{
-		using _traits = make_traits<floating_point, ::sqlpp::tag::expression, ::sqlpp::tag::named_expression>;
+		using _traits = make_traits<floating_point, ::sqlpp::tag::is_expression, ::sqlpp::tag::is_named_expression>;
 		using _recursive_traits = make_recursive_traits<Expr>;
 
 		static_assert(is_noop<Flag>::value or std::is_same<sqlpp::distinct_t, Flag>::value, "avg() used with flag other than 'distinct'");

include/sqlpp11/boolean.h

@@ -28,7 +28,6 @@
 #define SQLPP_BOOLEAN_H
 
 #include <cstdlib>
-#include <cassert>
 #include <ostream>
 #include <sqlpp11/basic_expression_operators.h>
 #include <sqlpp11/type_traits.h>
@@ -42,7 +41,8 @@ namespace sqlpp
 		// boolean value type
 		struct boolean
 		{
-			using _tag = ::sqlpp::tag::boolean;
+			using _traits = make_traits<boolean, ::sqlpp::tag::is_boolean, ::sqlpp::tag::is_expression>;
+			using _tag = ::sqlpp::tag::is_boolean;
 			using _cpp_value_type = bool;
 
 			struct _parameter_t
@@ -96,10 +96,46 @@ namespace sqlpp
 				bool _is_null;
 			};
 
-			template<typename Db, bool NullIsTrivial = false>
+			template<typename Db, typename FieldSpec>
-				struct _result_entry_t
+				struct _result_field_t;
+
+			// I am SO waiting for concepts lite!
+			template<typename Field, typename Enable = void>
+				struct field_methods_t
 				{
-					_result_entry_t():
+					static constexpr bool _null_is_trivial = true;
+					operator _cpp_value_type() const { return static_cast<const Field&>(*this).value(); }
+				};
+
+			template<typename Db, typename FieldSpec>
+				struct field_methods_t<
+						_result_field_t<Db, FieldSpec>, 
+				    typename std::enable_if<enforce_null_result_treatment_t<Db>::value 
+							and column_spec_can_be_null_t<FieldSpec>::value
+							and not null_is_trivial_value_t<FieldSpec>::value>::type>
+				{
+					static constexpr bool _null_is_trivial = false;
+				};
+
+			template<typename Db, typename FieldSpec>
+				struct _result_field_t: public field_methods_t<_result_field_t<Db, FieldSpec>>
+				{
+					using _field_methods_t = field_methods_t<_result_field_t<Db, FieldSpec>>;
+
+					using _traits = make_traits<integral,
+								tag_if<tag::null_is_trivial_value, _field_methods_t::_null_is_trivial>>;
+
+					struct _recursive_traits
+					{
+						using _parameters = std::tuple<>;
+						using _provided_tables = detail::type_set<>;
+						using _provided_outer_tables = detail::type_set<>;
+						using _required_tables = detail::type_set<>;
+						using _extra_tables = detail::type_set<>;
+						using _can_be_null = column_spec_can_be_null_t<FieldSpec>;
+					};
+
+					_result_field_t():
 						_is_valid(false),
 						_is_null(true),
 						_value(false)
@@ -119,33 +155,30 @@ namespace sqlpp
 
 					bool is_null() const
 					{ 
-						if (connector_assert_result_validity_t<Db>::value)
+						if (not _is_valid)
-							assert(_is_valid);
-						else if (not _is_valid)
 							throw exception("accessing is_null in non-existing row");
 						return _is_null; 
 					}
 
 					_cpp_value_type value() const
 					{
-						const bool null_value = _is_null and not NullIsTrivial and not connector_null_result_is_trivial_value_t<Db>::value;
+						if (not _is_valid)
-						if (connector_assert_result_validity_t<Db>::value)
+							throw exception("accessing value in non-existing row");
+
+						if (_is_null)
 						{
-							assert(_is_valid);
+							if (enforce_null_result_treatment_t<Db>::value and not null_is_trivial_value_t<FieldSpec>::value)
-							assert(not null_value);
+							{
-						}
-						else
-						{
-							if (not _is_valid)
-								throw exception("accessing value in non-existing row");
-							if (null_value)
 								throw exception("accessing value of NULL field");
+							}
+							else
+							{
+								return false;
+							}
 						}
 						return _value;
 					}
 
-					operator _cpp_value_type() const { return value(); }
-
 					template<typename Target>
 						void _bind(Target& target, size_t i)
 						{
@@ -200,8 +233,8 @@ namespace sqlpp
 				};
 		};
 
-		template<typename Db, bool TrivialIsNull>
+		template<typename Db, typename FieldSpec>
-			inline std::ostream& operator<<(std::ostream& os, const boolean::_result_entry_t<Db, TrivialIsNull>& e)
+			inline std::ostream& operator<<(std::ostream& os, const boolean::_result_field_t<Db, FieldSpec>& e)
 			{
 				return os << e.value();
 			}

include/sqlpp11/column.h

@@ -42,25 +42,31 @@
 namespace sqlpp
 {
 	template<typename Table, typename ColumnSpec>
-		struct column_t: public ColumnSpec::_value_type::template expression_operators<column_t<Table, ColumnSpec>>,
+		struct column_t: public value_type_of<ColumnSpec>::template expression_operators<column_t<Table, ColumnSpec>>,
-		public ColumnSpec::_value_type::template column_operators<column_t<Table, ColumnSpec>>
+		public value_type_of<ColumnSpec>::template column_operators<column_t<Table, ColumnSpec>>
 	{ 
-		using _traits = make_traits<typename ColumnSpec::_value_type, tag::column, tag::expression, tag::named_expression>;
+		struct _traits
+		{
+			using _value_type = value_type_of<ColumnSpec>;
+			using _tags = detail::make_joined_set_t<detail::type_set<tag::is_column, tag::is_expression, tag::is_named_expression>, typename ColumnSpec::_traits::_tags>;
+		};
+
 		struct _recursive_traits
 		{
 			using _parameters = std::tuple<>;
 			using _provided_tables = detail::type_set<>;
+			using _provided_outer_tables = detail::type_set<>;
 			using _required_tables = detail::type_set<Table>;
 			using _extra_tables = detail::type_set<>;
+			using _can_be_null = column_spec_can_be_null_t<ColumnSpec>;
 		};
 
 		using _spec_t = ColumnSpec;
 		using _table = Table;
-		using _column_type = typename _spec_t::_column_type;
 		using _name_t = typename _spec_t::_name_t;
 
 		template<typename T>
-			using _is_valid_operand = typename ColumnSpec::_value_type::template _is_valid_operand<T>;
+			using _is_valid_operand = typename value_type_of<ColumnSpec>::template _is_valid_operand<T>;
 
 		column_t() = default;
 		column_t(const column_t&) = default;

include/sqlpp11/concat.h

@@ -39,7 +39,7 @@ namespace sqlpp
 		struct concat_t: public value_type_of<First>::template expression_operators<concat_t<First, Args...>>,
 		public alias_operators<concat_t<First, Args...>>
 	{
-		using _traits = make_traits<value_type_of<First>, ::sqlpp::tag::expression, ::sqlpp::tag::named_expression>;
+		using _traits = make_traits<value_type_of<First>, ::sqlpp::tag::is_expression, ::sqlpp::tag::is_named_expression>;
 		using _recursive_traits = make_recursive_traits<First, Args...>;
 
 		static_assert(sizeof...(Args) > 0, "concat requires two arguments at least");

include/sqlpp11/count.h

@@ -36,7 +36,7 @@ namespace sqlpp
 		struct count_t: public sqlpp::detail::integral::template expression_operators<count_t<Flag, Expr>>,
 		public alias_operators<count_t<Flag, Expr>>
 	{
-		using _traits = make_traits<::sqlpp::detail::integral, ::sqlpp::tag::expression, ::sqlpp::tag::named_expression>;
+		using _traits = make_traits<::sqlpp::detail::integral, ::sqlpp::tag::is_expression, ::sqlpp::tag::is_named_expression>;
 		using _recursive_traits = make_recursive_traits<Expr>;
 
 		static_assert(is_noop<Flag>::value or std::is_same<sqlpp::distinct_t, Flag>::value, "count() used with flag other than 'distinct'");

include/sqlpp11/default_value.h

@@ -33,7 +33,7 @@ namespace sqlpp
 {
 	struct default_value_t
 	{
-		using _traits = make_traits<no_value_t, tag::expression>;
+		using _traits = make_traits<no_value_t, tag::is_expression>;
 		using _recursive_traits = make_recursive_traits<>;
 
 		static constexpr bool _is_trivial() { return false; }

include/sqlpp11/detail/column_index_sequence.h

@@ -49,7 +49,7 @@ namespace sqlpp
 			};
 
 		template<std::size_t LastIndex, std::size_t... Ints, typename AliasProvider, typename... Fields, typename... Rest>
-			struct make_column_index_sequence_impl<column_index_sequence<LastIndex, Ints...>, multi_field_t<AliasProvider, Fields...>, Rest...>
+			struct make_column_index_sequence_impl<column_index_sequence<LastIndex, Ints...>, multi_field_spec_t<AliasProvider, Fields...>, Rest...>
 			{
 				using type = typename make_column_index_sequence_impl<column_index_sequence<LastIndex + sizeof...(Fields), Ints..., LastIndex + sizeof...(Fields)>, Rest...>::type;
 			};

include/sqlpp11/detail/type_set.h

@@ -204,13 +204,30 @@ namespace sqlpp
 			struct make_difference_set<type_set<Minuends...>, type_set<Subtrahends...>>
 			{
 				template<typename E>
-				using is_subtrahend = is_element_of<E, type_set<Subtrahends...>>;
+					using is_subtrahend = is_element_of<E, type_set<Subtrahends...>>;
 				using type = typename make_type_set_if_not<is_subtrahend, Minuends...>::type;
 			};
 
 		template<typename Minuend, typename Subtrahend>
 			using make_difference_set_t = typename make_difference_set<Minuend, Subtrahend>::type;
 
+		template<typename Lhs, typename Rhs>
+			struct make_intersect_set
+			{
+				static_assert(::sqlpp::wrong_t<Lhs, Rhs>::value, "invalid argument for intersect set");
+			};
+
+		template<typename... LhsElements, typename... RhsElements>
+			struct make_intersect_set<type_set<LhsElements...>, type_set<RhsElements...>>
+			{
+				template<typename E>
+					using is_in_both = is_element_of<E, make_type_set_t<LhsElements..., RhsElements...>>;
+				using type = typename make_type_set_if<is_in_both, LhsElements...>::type;
+			};
+
+		template<typename Lhs, typename Rhs>
+			using make_intersect_set_t = typename make_intersect_set<Lhs, Rhs>::type;
+
 
 		template<template<typename> class Transformation, typename T>
 			struct transform_set

include/sqlpp11/exists.h

@@ -35,7 +35,7 @@ namespace sqlpp
 		struct exists_t: public boolean::template expression_operators<exists_t<Select>>,
 		public alias_operators<exists_t<Select>>
 	{
-		using _traits = make_traits<boolean, ::sqlpp::tag::expression, ::sqlpp::tag::named_expression>;
+		using _traits = make_traits<boolean, ::sqlpp::tag::is_expression, ::sqlpp::tag::is_named_expression>;
 		using _recursive_traits = make_recursive_traits<Select>;
 
 		static_assert(is_select_t<Select>::value, "exists() requires a select expression as argument");

include/sqlpp11/expression.h

@@ -41,10 +41,12 @@ namespace sqlpp
 		struct binary_expression_t<Lhs, op::equal_to, Rhs>: public ::sqlpp::detail::boolean::template expression_operators<binary_expression_t<Lhs, op::equal_to, Rhs>>,
 		public alias_operators<binary_expression_t<Lhs, op::equal_to, Rhs>>
 		{
-			using _traits = make_traits<boolean, sqlpp::tag::expression>;
+			using _traits = make_traits<boolean, sqlpp::tag::is_expression>;
 			using _recursive_traits = make_recursive_traits<Lhs, Rhs>;
+			using _lhs_t = Lhs;
+			using _rhs_t = allow_tvin_t<Rhs>;
 
-			binary_expression_t(Lhs lhs, Rhs rhs):
+			binary_expression_t(_lhs_t lhs, _rhs_t rhs):
 				_lhs(lhs), 
 				_rhs(rhs)
 			{}
@@ -55,8 +57,8 @@ namespace sqlpp
 			binary_expression_t& operator=(binary_expression_t&&) = default;
 			~binary_expression_t() = default;
 
-			Lhs _lhs;
+			_lhs_t _lhs;
-			maybe_tvin_t<Rhs> _rhs;
+			_rhs_t _rhs;
 		};
 
 	template<typename Context, typename Lhs, typename Rhs>
@@ -68,7 +70,7 @@ namespace sqlpp
 			{
 				context << "(";
 				serialize(t._lhs, context);
-				if (t._rhs._is_trivial())
+				if (rhs_is_null(t))
 				{
 					context << " IS NULL";
 				}
@@ -86,10 +88,12 @@ namespace sqlpp
 		struct binary_expression_t<Lhs, op::not_equal_to, Rhs>: public ::sqlpp::detail::boolean::template expression_operators<binary_expression_t<Lhs, op::not_equal_to, Rhs>>,
 		public alias_operators<binary_expression_t<Lhs, op::not_equal_to, Rhs>>
 		{
-			using _traits = make_traits<boolean, sqlpp::tag::expression>;
+			using _traits = make_traits<boolean, sqlpp::tag::is_expression>;
 			using _recursive_traits = make_recursive_traits<Lhs, Rhs>;
+			using _lhs_t = Lhs;
+			using _rhs_t = allow_tvin_t<Rhs>;
 
-			binary_expression_t(Lhs lhs, Rhs rhs):
+			binary_expression_t(Lhs lhs, _rhs_t rhs):
 				_lhs(lhs), 
 				_rhs(rhs)
 			{}
@@ -100,8 +104,8 @@ namespace sqlpp
 			binary_expression_t& operator=(binary_expression_t&&) = default;
 			~binary_expression_t() = default;
 
-			Lhs _lhs;
+			_lhs_t _lhs;
-			maybe_tvin_t<Rhs> _rhs;
+			_rhs_t _rhs;
 		};
 
 	template<typename Context, typename Lhs, typename Rhs>
@@ -113,7 +117,7 @@ namespace sqlpp
 			{
 				context << "(";
 				serialize(t._lhs, context);
-				if (t._rhs._is_trivial())
+				if (rhs_is_null(t))
 				{
 					context << " IS NOT NULL";
 				}
@@ -131,7 +135,7 @@ namespace sqlpp
 		struct unary_expression_t<op::logical_not, Rhs>: public ::sqlpp::detail::boolean::template expression_operators<unary_expression_t<op::logical_not, Rhs>>,
 		public alias_operators<unary_expression_t<op::logical_not, Rhs>>
 		{
-			using _traits = make_traits<boolean, sqlpp::tag::expression>;
+			using _traits = make_traits<boolean, sqlpp::tag::is_expression>;
 			using _recursive_traits = make_recursive_traits<Rhs>;
 
 			unary_expression_t(Rhs rhs):
@@ -155,10 +159,17 @@ namespace sqlpp
 			static Context& _(const T& t, Context& context)
 			{
 				context << "(";
-				context << "NOT ";
+				if (trivial_value_is_null_t<Rhs>::value)
-				serialize(t._lhs, context);
+				{
+					serialize(t._lhs, context);
+					context << " IS NULL ";
+				}
+				else
+				{
+					context << "NOT ";
+					serialize(t._rhs, context);
+				}
 				context << ")";
-				return context;
 			}
 		};
 
@@ -166,7 +177,7 @@ namespace sqlpp
 		struct binary_expression_t: public value_type_of<O>::template expression_operators<binary_expression_t<Lhs, O, Rhs>>,
 		public alias_operators<binary_expression_t<Lhs, O, Rhs>>
 	{
-		using _traits = make_traits<value_type_of<O>, sqlpp::tag::expression>;
+		using _traits = make_traits<value_type_of<O>, sqlpp::tag::is_expression>;
 		using _recursive_traits = make_recursive_traits<Lhs, Rhs>;
 
 		binary_expression_t(Lhs lhs, Rhs rhs):
@@ -204,7 +215,7 @@ namespace sqlpp
 		struct unary_expression_t: public value_type_of<O>::template expression_operators<unary_expression_t<O, Rhs>>,
 		public alias_operators<unary_expression_t<O, Rhs>>
 	{
-		using _traits = make_traits<value_type_of<O>, sqlpp::tag::expression>;
+		using _traits = make_traits<value_type_of<O>, sqlpp::tag::is_expression>;
 		using _recursive_traits = make_recursive_traits<Rhs>;
 
 		unary_expression_t(Rhs rhs):

include/sqlpp11/extra_tables.h

@@ -51,13 +51,15 @@ namespace sqlpp
 	template<typename... Tables>
 		struct extra_tables_t
 		{
-			using _traits = make_traits<no_value_t, ::sqlpp::tag::extra_tables>;
+			using _traits = make_traits<no_value_t, ::sqlpp::tag::is_extra_tables>;
 			struct _recursive_traits
 			{
 				using _parameters = std::tuple<>;
 				using _required_tables = ::sqlpp::detail::type_set<>;
+				using _provided_outer_tables = ::sqlpp::detail::type_set<>;
 				using _provided_tables = ::sqlpp::detail::type_set<>;
 				using _extra_tables = ::sqlpp::detail::type_set<Tables...>;
+				using _can_be_null = std::false_type;
 			};
 
 			// FIXME: extra_tables must not require tables!
@@ -104,7 +106,7 @@ namespace sqlpp
 	// NO EXTRA TABLES YET
 	struct no_extra_tables_t
 	{
-		using _traits = make_traits<no_value_t, ::sqlpp::tag::noop>;
+		using _traits = make_traits<no_value_t, ::sqlpp::tag::is_noop>;
 		using _recursive_traits = make_recursive_traits<>;
 
 		// Data

include/sqlpp11/field_spec.h

@@ -0,0 +1,77 @@
+/*
+ * Copyright (c) 2013-2014, Roland Bock
+ * All rights reserved.
+ * 
+ * Redistribution and use in source and binary forms, with or without modification,
+ * are permitted provided that the following conditions are met:
+ * 
+ *   Redistributions of source code must retain the above copyright notice, this
+ *   list of conditions and the following disclaimer.
+ * 
+ *   Redistributions in binary form must reproduce the above copyright notice, this
+ *   list of conditions and the following disclaimer in the documentation and/or
+ *   other materials provided with the distribution.
+ * 
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+ * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR
+ * ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+ * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+ * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
+ * ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+ * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#ifndef SQLPP_FIELD_SPEC_H
+#define SQLPP_FIELD_SPEC_H
+
+#include <sqlpp11/multi_column.h>
+
+namespace sqlpp
+{
+	template<typename NameType, typename ValueType, bool CanBeNull, bool NullIsTrivialValue>
+		struct field_spec_t
+		{ 
+			using _traits = make_traits<ValueType, tag::is_noop, 
+						tag_if<tag::can_be_null, CanBeNull>,
+						tag_if<tag::null_is_trivial_value, NullIsTrivialValue>
+							>;
+			using _recursive_traits = make_recursive_traits<>;
+
+			using _name_t = NameType;
+		};
+
+	template<typename AliasProvider, typename FieldSpecTuple>
+		struct multi_field_spec_t
+		{
+		};
+
+	namespace detail
+	{
+		template<typename Select, typename NamedExpr>
+			struct make_field_spec_impl
+			{
+				static constexpr bool _can_be_null = can_be_null_t<NamedExpr>::value;
+				static constexpr bool _depends_on_outer_table = detail::make_intersect_set_t<required_tables_of<NamedExpr>, typename Select::_used_outer_tables>::size::value > 0;
+
+				using type = field_spec_t<typename NamedExpr::_name_t, 
+							value_type_of<NamedExpr>,
+							detail::any_t<_can_be_null, _depends_on_outer_table>::value,
+							null_is_trivial_value_t<NamedExpr>::value>;
+			};
+
+		template<typename Select, typename AliasProvider, typename... NamedExprs>
+			struct make_field_spec_impl<Select, multi_column_alias_t<AliasProvider, NamedExprs...>>
+			{
+				using type = multi_field_spec_t<AliasProvider, std::tuple<typename make_field_spec_impl<Select, NamedExprs>::type...>>;
+			};
+	}
+
+	template<typename Select, typename NamedExpr>
+		using make_field_spec_t = typename detail::make_field_spec_impl<Select, NamedExpr>::type;
+
+}
+
+#endif

include/sqlpp11/floating_point.h

@@ -28,7 +28,6 @@
 #define SQLPP_FLOATING_POINT_H
 
 #include <cstdlib>
-#include <cassert>
 #include <sqlpp11/basic_expression_operators.h>
 #include <sqlpp11/type_traits.h>
 #include <sqlpp11/exception.h>
@@ -41,7 +40,8 @@ namespace sqlpp
 		// floating_point value type
 		struct floating_point
 		{
-			using _tag = ::sqlpp::tag::floating_point;
+			using _traits = make_traits<floating_point, ::sqlpp::tag::is_floating_point, ::sqlpp::tag::is_expression>;
+			using _tag = ::sqlpp::tag::is_floating_point;
 			using _cpp_value_type = double;
 
 			struct _parameter_t
@@ -95,12 +95,46 @@ namespace sqlpp
 				bool _is_null;
 			};
 
-			template<typename Db, bool NullIsTrivial = false>
+			template<typename Db, typename FieldSpec>
-				struct _result_entry_t
+				struct _result_field_t;
-				{
-					using _value_type = integral;
 
-					_result_entry_t():
+			// I am SO waiting for concepts lite!
+			template<typename Field, typename Enable = void>
+				struct field_methods_t
+				{
+					static constexpr bool _null_is_trivial = true;
+					operator _cpp_value_type() const { return static_cast<const Field&>(*this).value(); }
+				};
+
+			template<typename Db, typename FieldSpec>
+				struct field_methods_t<
+						_result_field_t<Db, FieldSpec>, 
+				    typename std::enable_if<enforce_null_result_treatment_t<Db>::value 
+							and column_spec_can_be_null_t<FieldSpec>::value
+							and not null_is_trivial_value_t<FieldSpec>::value>::type>
+				{
+					static constexpr bool _null_is_trivial = false;
+				};
+
+			template<typename Db, typename FieldSpec>
+				struct _result_field_t: public field_methods_t<_result_field_t<Db, FieldSpec>>
+				{
+					using _field_methods_t = field_methods_t<_result_field_t<Db, FieldSpec>>;
+
+					using _traits = make_traits<integral,
+								tag_if<tag::null_is_trivial_value, _field_methods_t::_null_is_trivial>>;
+
+					struct _recursive_traits
+					{
+						using _parameters = std::tuple<>;
+						using _provided_tables = detail::type_set<>;
+						using _provided_outer_tables = detail::type_set<>;
+						using _required_tables = detail::type_set<>;
+						using _extra_tables = detail::type_set<>;
+						using _can_be_null = column_spec_can_be_null_t<FieldSpec>;
+					};
+
+					_result_field_t():
 						_is_valid(false),
 						_is_null(true),
 						_value(0)
@@ -120,33 +154,30 @@ namespace sqlpp
 
 					bool is_null() const
 					{ 
-						if (connector_assert_result_validity_t<Db>::value)
+						if (not _is_valid)
-							assert(_is_valid);
-						else if (not _is_valid)
 							throw exception("accessing is_null in non-existing row");
 						return _is_null; 
 					}
 
 					_cpp_value_type value() const
 					{
-						const bool null_value = _is_null and not NullIsTrivial and not connector_null_result_is_trivial_value_t<Db>::value;
+						if (not _is_valid)
-						if (connector_assert_result_validity_t<Db>::value)
+							throw exception("accessing value in non-existing row");
+
+						if (_is_null)
 						{
-							assert(_is_valid);
+							if (enforce_null_result_treatment_t<Db>::value and not null_is_trivial_value_t<FieldSpec>::value)
-							assert(not null_value);
+							{
-						}
-						else
-						{
-							if (not _is_valid)
-								throw exception("accessing value in non-existing row");
-							if (null_value)
 								throw exception("accessing value of NULL field");
+							}
+							else
+							{
+								return 0;
+							}
 						}
 						return _value;
 					}
 
-					operator _cpp_value_type() const { return value(); }
-
 					template<typename Target>
 						void _bind(Target& target, size_t i)
 						{
@@ -257,8 +288,8 @@ namespace sqlpp
 				};
 		};
 
-		template<typename Db, bool TrivialIsNull>
+		template<typename Db, typename FieldSpec>
-			inline std::ostream& operator<<(std::ostream& os, const floating_point::_result_entry_t<Db, TrivialIsNull>& e)
+			inline std::ostream& operator<<(std::ostream& os, const floating_point::_result_field_t<Db, FieldSpec>& e)
 			{
 				return os << e.value();
 			}

include/sqlpp11/from.h

@@ -59,7 +59,7 @@ namespace sqlpp
 	template<typename Database, typename... Tables>
 		struct from_t
 		{
-			using _traits = make_traits<no_value_t, ::sqlpp::tag::from>;
+			using _traits = make_traits<no_value_t, ::sqlpp::tag::is_from>;
 			using _recursive_traits = make_recursive_traits<Tables...>;
 			using _is_dynamic = is_database<Database>;
 
@@ -125,7 +125,7 @@ namespace sqlpp
 
 	struct no_from_t
 	{
-		using _traits = make_traits<no_value_t, ::sqlpp::tag::noop>;
+		using _traits = make_traits<no_value_t, ::sqlpp::tag::is_noop>;
 		using _recursive_traits = make_recursive_traits<>;
 
 		// Data

include/sqlpp11/functions.h

@@ -56,8 +56,11 @@ namespace sqlpp
 		struct verbatim_t: public ValueType::template expression_operators<verbatim_t<ValueType>>,
 		public alias_operators<verbatim_t<ValueType>>
 	{
-		using _traits = make_traits<ValueType, ::sqlpp::tag::expression>;
+		using _traits = make_traits<ValueType, ::sqlpp::tag::is_expression>;
-		using _recursive_traits = make_recursive_traits<>;
+		struct _recursive_traits : public make_recursive_traits<>
+		{
+			using _can_be_null = std::true_type; // since we do not know what's going on inside the verbatim, we assume it can be null
+		};
 
 		verbatim_t(std::string verbatim): _verbatim(verbatim) {}
 		verbatim_t(const verbatim_t&) = default;
@@ -99,7 +102,7 @@ namespace sqlpp
 	template<typename Container>
 		struct value_list_t // to be used in .in() method
 		{
-			using _traits = make_traits<value_type_t<typename Container::value_type>, ::sqlpp::tag::expression>;
+			using _traits = make_traits<value_type_t<typename Container::value_type>, ::sqlpp::tag::is_expression>;
 			using _recursive_traits = make_recursive_traits<>;
 
 			using _container_t = Container;

include/sqlpp11/group_by.h

@@ -59,7 +59,7 @@ namespace sqlpp
 	template<typename Database, typename... Expressions>
 		struct group_by_t
 		{
-			using _traits = make_traits<no_value_t, ::sqlpp::tag::group_by>;
+			using _traits = make_traits<no_value_t, ::sqlpp::tag::is_group_by>;
 			using _recursive_traits = make_recursive_traits<Expressions...>;
 
 			using _is_dynamic = is_database<Database>;
@@ -135,7 +135,7 @@ namespace sqlpp
 	// NO GROUP BY YET
 	struct no_group_by_t
 	{
-		using _traits = make_traits<no_value_t, ::sqlpp::tag::noop>;
+		using _traits = make_traits<no_value_t, ::sqlpp::tag::is_noop>;
 		using _recursive_traits = make_recursive_traits<>;
 
 		// Data

include/sqlpp11/having.h

@@ -58,7 +58,7 @@ namespace sqlpp
 	template<typename Database, typename... Expressions>
 		struct having_t
 		{
-			using _traits = make_traits<no_value_t, ::sqlpp::tag::having>;
+			using _traits = make_traits<no_value_t, ::sqlpp::tag::is_having>;
 			using _recursive_traits = make_recursive_traits<Expressions...>;
 
 			using _is_dynamic = is_database<Database>;
@@ -133,7 +133,7 @@ namespace sqlpp
 	// NO HAVING YET
 	struct no_having_t
 	{
-		using _traits = make_traits<no_value_t, ::sqlpp::tag::noop>;
+		using _traits = make_traits<no_value_t, ::sqlpp::tag::is_noop>;
 		using _recursive_traits = make_recursive_traits<>;
 
 		// Data

include/sqlpp11/in.h

@@ -38,7 +38,7 @@ namespace sqlpp
 		struct in_t: public boolean::template expression_operators<in_t<NotInverted, Operand, Args...>>,
 		public alias_operators<in_t<NotInverted, Operand, Args...>>
 	{
-		using _traits = make_traits<boolean, ::sqlpp::tag::expression, ::sqlpp::tag::named_expression>;
+		using _traits = make_traits<boolean, ::sqlpp::tag::is_expression, ::sqlpp::tag::is_named_expression>;
 		using _recursive_traits = make_recursive_traits<Operand, Args...>;
 
 		static constexpr bool _inverted = not NotInverted;

include/sqlpp11/insert.h

@@ -42,7 +42,7 @@ namespace sqlpp
 
 	struct insert_t: public statement_name_t<insert_name_t>
 	{
-		using _traits = make_traits<no_value_t, tag::return_value>;
+		using _traits = make_traits<no_value_t, tag::is_return_value>;
 		struct _name_t {};
 
 		template<typename Policies>

include/sqlpp11/insert_value.h

@@ -49,7 +49,7 @@ namespace sqlpp
 			{
 				struct type
 				{
-					using _traits = make_traits<no_value_t, ::sqlpp::tag::noop>;
+					using _traits = make_traits<no_value_t, ::sqlpp::tag::is_noop>;
 					using _recursive_traits = make_recursive_traits<>;
 				};
 			};
@@ -61,7 +61,7 @@ namespace sqlpp
 			using _is_insert_value = std::true_type;
 			using _pure_value_t = typename value_type_of<Column>::_cpp_value_type;
 			using _wrapped_value_t = typename wrap_operand<_pure_value_t>::type;
-			using _tvin_t = typename detail::type_if<tvin_t<_wrapped_value_t>, can_be_null_t<Column>::value>::type; // static asserts and SFINAE do not work together
+			using _tvin_t = typename detail::type_if<tvin_arg_t<_wrapped_value_t>, can_be_null_t<Column>::value>::type; // static asserts and SFINAE do not work together
 			using _null_t = typename detail::type_if<null_t, can_be_null_t<Column>::value>::type; // static asserts and SFINAE do not work together
 
 			insert_value_t(assignment_t<Column, _wrapped_value_t> assignment):

include/sqlpp11/insert_value_list.h

@@ -97,8 +97,8 @@ namespace sqlpp
 			~insert_list_data_t() = default;
 
 			std::tuple<Assignments...> _assignments; // FIXME: Need to replace _columns and _values by _assignments (connector-container requires assignments)
-			std::tuple<simple_column_t<typename Assignments::_column_t>...> _columns;
+			std::tuple<simple_column_t<typename Assignments::_lhs_t>...> _columns;
-			std::tuple<typename Assignments::_value_t...> _values;
+			std::tuple<typename Assignments::_rhs_t...> _values;
 			interpretable_list_t<Database> _dynamic_columns;
 			interpretable_list_t<Database> _dynamic_values;
 		};
@@ -106,8 +106,8 @@ namespace sqlpp
 	template<typename Database, typename... Assignments>
 		struct insert_list_t
 		{
-			using _traits = make_traits<no_value_t, ::sqlpp::tag::insert_list>;
+			using _traits = make_traits<no_value_t, ::sqlpp::tag::is_insert_list>;
-			using _recursive_traits = make_recursive_traits<typename Assignments::_column_t..., typename Assignments::_value_t...>;
+			using _recursive_traits = make_recursive_traits<typename Assignments::_lhs_t..., typename Assignments::_rhs_t...>;
 
 			using _is_dynamic = is_database<Database>;
 
@@ -134,9 +134,9 @@ namespace sqlpp
 						{
 							static_assert(_is_dynamic::value, "add must not be called for static from()");
 							static_assert(is_assignment_t<Assignment>::value, "add() arguments require to be assigments");
-							using _assigned_columns = detail::make_type_set_t<typename Assignments::_column_t...>;
+							using _assigned_columns = detail::make_type_set_t<typename Assignments::_lhs_t...>;
-							static_assert(not detail::is_element_of<typename Assignment::_column_t, _assigned_columns>::value, "Must not assign value to column twice");
+							static_assert(not detail::is_element_of<typename Assignment::_lhs_t, _assigned_columns>::value, "Must not assign value to column twice");
-							static_assert(not must_not_insert_t<typename Assignment::_column_t>::value, "add() argument must not be used in insert");
+							static_assert(not must_not_insert_t<typename Assignment::_lhs_t>::value, "add() argument must not be used in insert");
 							static_assert(not TableCheckRequired::value or Policies::template _no_unknown_tables<Assignment>::value, "add() contains a column from a foreign table");
 
 							using ok = ::sqlpp::detail::all_t<
@@ -150,7 +150,7 @@ namespace sqlpp
 					template<typename Assignment>
 						void _add_impl(Assignment assignment, const std::true_type&)
 						{
-							_data._dynamic_columns.emplace_back(simple_column_t<typename Assignment::_column_t>{assignment._lhs});
+							_data._dynamic_columns.emplace_back(simple_column_t<typename Assignment::_lhs_t>{assignment._lhs});
 							_data._dynamic_values.emplace_back(assignment._rhs);
 						}
 
@@ -207,7 +207,7 @@ namespace sqlpp
 	template<typename... Columns>
 		struct column_list_t
 		{
-			using _traits = make_traits<no_value_t, ::sqlpp::tag::column_list>;
+			using _traits = make_traits<no_value_t, ::sqlpp::tag::is_column_list>;
 			using _recursive_traits = make_recursive_traits<Columns...>;
 
 			static_assert(sizeof...(Columns), "at least one column required in columns()");
@@ -233,7 +233,7 @@ namespace sqlpp
 						void add(Assignments... assignments)
 						{
 							static_assert(::sqlpp::detail::all_t<is_assignment_t<Assignments>::value...>::value, "add_values() arguments have to be assignments");
-							using _arg_value_tuple = std::tuple<insert_value_t<typename Assignments::_column_t>...>;
+							using _arg_value_tuple = std::tuple<insert_value_t<typename Assignments::_lhs_t>...>;
 							using _args_correct = std::is_same<_arg_value_tuple, _value_tuple_t>;
 							static_assert(_args_correct::value, "add_values() arguments do not match columns() arguments");
 
@@ -248,7 +248,7 @@ namespace sqlpp
 					template<typename... Assignments>
 						void _add_impl(const std::true_type&, Assignments... assignments)
 						{
-							return _data._insert_values.emplace_back(insert_value_t<typename Assignments::_column_t>{assignments}...);
+							return _data._insert_values.emplace_back(insert_value_t<typename Assignments::_lhs_t>{assignments}...);
 						}
 
 					template<typename... Assignments>
@@ -285,7 +285,7 @@ namespace sqlpp
 	// NO INSERT COLUMNS/VALUES YET
 	struct no_insert_value_list_t
 	{
-		using _traits = make_traits<no_value_t, ::sqlpp::tag::noop>;
+		using _traits = make_traits<no_value_t, ::sqlpp::tag::is_noop>;
 		using _recursive_traits = make_recursive_traits<>;
 
 		// Data
@@ -359,9 +359,9 @@ namespace sqlpp
 					{
 						static_assert(not ::sqlpp::detail::has_duplicates<Assignments...>::value, "at least one duplicate argument detected in set()");
 						static_assert(sqlpp::detail::all_t<is_assignment_t<Assignments>::value...>::value, "at least one argument is not an assignment in set()");
-						static_assert(sqlpp::detail::none_t<must_not_insert_t<typename Assignments::_column_t>::value...>::value, "at least one assignment is prohibited by its column definition in set()");
+						static_assert(sqlpp::detail::none_t<must_not_insert_t<typename Assignments::_lhs_t>::value...>::value, "at least one assignment is prohibited by its column definition in set()");
 
-						using _column_required_tables = typename ::sqlpp::detail::make_joined_set<required_tables_of<typename Assignments::_column_t>...>::type;
+						using _column_required_tables = ::sqlpp::detail::make_joined_set_t<required_tables_of<typename Assignments::_lhs_t>...>;
 						static_assert(sizeof...(Assignments) ? (_column_required_tables::size::value == 1) : true, "set() contains assignments for columns from several tables");
 
 						return { *static_cast<typename Policies::_statement_t*>(this), insert_list_data_t<Database, Assignments...>{assignments...} };

include/sqlpp11/integral.h

@@ -28,7 +28,6 @@
 #define SQLPP_INTEGRAL_H
 
 #include <cstdlib>
-#include <cassert>
 #include <sqlpp11/basic_expression_operators.h>
 #include <sqlpp11/type_traits.h>
 #include <sqlpp11/exception.h>
@@ -43,8 +42,8 @@ namespace sqlpp
 		// integral value type
 		struct integral
 		{
-			using _traits = make_traits<integral, ::sqlpp::tag::expression>;
+			using _traits = make_traits<integral, ::sqlpp::tag::is_integral, ::sqlpp::tag::is_expression>;
-			using _tag = ::sqlpp::tag::integral;
+			using _tag = ::sqlpp::tag::is_integral;
 			using _cpp_value_type = int64_t;
 
 			struct _parameter_t
@@ -97,12 +96,46 @@ namespace sqlpp
 				bool _is_null;
 			};
 
-			template<typename Db, bool NullIsTrivial = false>
+			template<typename Db, typename FieldSpec>
-				struct _result_entry_t
+				struct _result_field_t;
-				{
-					using _value_type = integral;
 
-					_result_entry_t():
+			// I am SO waiting for concepts lite!
+			template<typename Field, typename Enable = void>
+				struct field_methods_t
+				{
+					static constexpr bool _null_is_trivial = true;
+					operator _cpp_value_type() const { return static_cast<const Field&>(*this).value(); }
+				};
+
+			template<typename Db, typename FieldSpec>
+				struct field_methods_t<
+						_result_field_t<Db, FieldSpec>, 
+				    typename std::enable_if<enforce_null_result_treatment_t<Db>::value 
+							and column_spec_can_be_null_t<FieldSpec>::value
+							and not null_is_trivial_value_t<FieldSpec>::value>::type>
+				{
+					static constexpr bool _null_is_trivial = false;
+				};
+
+			template<typename Db, typename FieldSpec>
+				struct _result_field_t: public field_methods_t<_result_field_t<Db, FieldSpec>>
+				{
+					using _field_methods_t = field_methods_t<_result_field_t<Db, FieldSpec>>;
+
+					using _traits = make_traits<integral,
+								tag_if<tag::null_is_trivial_value, _field_methods_t::_null_is_trivial>>;
+
+					struct _recursive_traits
+					{
+						using _parameters = std::tuple<>;
+						using _provided_tables = detail::type_set<>;
+						using _provided_outer_tables = detail::type_set<>;
+						using _required_tables = detail::type_set<>;
+						using _extra_tables = detail::type_set<>;
+						using _can_be_null = column_spec_can_be_null_t<FieldSpec>;
+					};
+
+					_result_field_t():
 						_is_valid(false),
 						_is_null(true),
 						_value(0)
@@ -122,33 +155,30 @@ namespace sqlpp
 
 					bool is_null() const
 					{ 
-						if (connector_assert_result_validity_t<Db>::value)
+						if (not _is_valid)
-							assert(_is_valid);
-						else if (not _is_valid)
 							throw exception("accessing is_null in non-existing row");
 						return _is_null; 
 					}
 
 					_cpp_value_type value() const
 					{
-						const bool null_value = _is_null and not NullIsTrivial and not connector_null_result_is_trivial_value_t<Db>::value;
+						if (not _is_valid)
-						if (connector_assert_result_validity_t<Db>::value)
+							throw exception("accessing value in non-existing row");
+
+						if (_is_null)
 						{
-							assert(_is_valid);
+							if (enforce_null_result_treatment_t<Db>::value and not null_is_trivial_value_t<FieldSpec>::value)
-							assert(not null_value);
+							{
-						}
-						else
-						{
-							if (not _is_valid)
-								throw exception("accessing value in non-existing row");
-							if (null_value)
 								throw exception("accessing value of NULL field");
+							}
+							else
+							{
+								return 0;
+							}
 						}
 						return _value;
 					}
 
-					operator _cpp_value_type() const { return value(); }
-
 					template<typename Target>
 						void _bind(Target& target, size_t i)
 						{
@@ -270,8 +300,8 @@ namespace sqlpp
 				};
 		};
 
-		template<typename Db, bool NullIsTrivial>
+		template<typename Db, typename FieldSpec>
-			inline std::ostream& operator<<(std::ostream& os, const integral::_result_entry_t<Db, NullIsTrivial>& e)
+			inline std::ostream& operator<<(std::ostream& os, const integral::_result_field_t<Db, FieldSpec>& e)
 			{
 				return os << e.value();
 			}

include/sqlpp11/into.h

@@ -57,7 +57,7 @@ namespace sqlpp
 	template<typename Database, typename Table>
 		struct into_t
 		{
-			using _traits = make_traits<no_value_t, ::sqlpp::tag::into>;
+			using _traits = make_traits<no_value_t, ::sqlpp::tag::is_into>;
 			using _recursive_traits = make_recursive_traits<Table>;
 
 			static_assert(is_table_t<Table>::value, "argument has to be a table");
@@ -103,7 +103,7 @@ namespace sqlpp
 	// NO INTO YET
 	struct no_into_t
 	{
-		using _traits = make_traits<no_value_t, ::sqlpp::tag::noop>;
+		using _traits = make_traits<no_value_t, ::sqlpp::tag::is_noop>;
 		using _recursive_traits = make_recursive_traits<>;
 
 		// Data

include/sqlpp11/is_null.h

@@ -37,7 +37,7 @@ namespace sqlpp
 		struct is_null_t: public boolean::template expression_operators<is_null_t<NotInverted, Operand>>,
 		public alias_operators<is_null_t<NotInverted, Operand>>
 	{
-		using _traits = make_traits<boolean, ::sqlpp::tag::expression, ::sqlpp::tag::named_expression>;
+		using _traits = make_traits<boolean, ::sqlpp::tag::is_expression, ::sqlpp::tag::is_named_expression>;
 		using _recursive_traits = make_recursive_traits<Operand>;
 
 		static constexpr bool _inverted = not NotInverted;

include/sqlpp11/join.h

@@ -35,41 +35,29 @@ namespace sqlpp
 {
 	struct inner_join_t
 	{
-		template<typename Db>
+		template<typename Lhs, typename Rhs>
-			struct _is_supported 
+			using _provided_outer_tables = detail::make_joined_set_t<provided_outer_tables_of<Lhs>, provided_outer_tables_of<Rhs>>;
-			{ 
-				static constexpr bool value = Db::_supports_inner_join; 
-			};
 
 		static constexpr const char* _name = " INNER ";
 	};
 	struct outer_join_t
 	{
-		template<typename Db>
+		template<typename Lhs, typename Rhs>
-			struct _is_supported 
+			using _provided_outer_tables = detail::make_joined_set_t<provided_tables_of<Lhs>, provided_tables_of<Rhs>>;
-			{ 
-				static constexpr bool value = Db::_supports_outer_join; 
-			};
 
 		static constexpr const char* _name = " OUTER ";
 	};
 	struct left_outer_join_t
 	{
-		template<typename Db>
+		template<typename Lhs, typename Rhs>
-			struct _is_supported 
+			using _provided_outer_tables = detail::make_joined_set_t<provided_outer_tables_of<Lhs>, provided_tables_of<Rhs>>;
-			{ 
-				static constexpr bool value = Db::_supports_left_outer_join; 
-			};
 
 		static constexpr const char* _name = " LEFT OUTER ";
 	};
 	struct right_outer_join_t
 	{
-		template<typename Db>
+		template<typename Lhs, typename Rhs>
-			struct _is_supported 
+			using _provided_outer_tables = detail::make_joined_set_t<provided_tables_of<Lhs>, provided_outer_tables_of<Rhs>>;
-			{ 
-				static constexpr bool value = Db::_supports_right_outer_join; 
-			};
 
 		static constexpr const char* _name = " RIGHT OUTER ";
 	};
@@ -77,8 +65,17 @@ namespace sqlpp
 	template<typename JoinType, typename Lhs, typename Rhs, typename On = noop>
 		struct join_t
 		{
-			using _traits = make_traits<no_value_t, tag::table, tag::join>;
+			using _traits = make_traits<no_value_t, tag::is_table, tag::is_join>;
-			using _recursive_traits = make_recursive_traits<Lhs, Rhs>;
+			struct _recursive_traits
+			{
+				using _required_tables = detail::make_joined_set_t<required_tables_of<Lhs>, required_tables_of<Rhs>>;
+				using _provided_tables = detail::make_joined_set_t<provided_tables_of<Lhs>, provided_tables_of<Rhs>>;
+				using _provided_outer_tables = typename JoinType::template _provided_outer_tables<Lhs, Rhs>;
+				using _extra_tables = detail::make_joined_set_t<extra_tables_of<Lhs>, extra_tables_of<Rhs>>;
+				using _parameters = detail::make_parameter_tuple_t<parameters_of<Lhs>, parameters_of<Rhs>>;
+				using _can_be_null = std::false_type;
+			};
+
 
 			static_assert(is_table_t<Lhs>::value, "lhs argument for join() has to be a table or join");
 			static_assert(is_table_t<Rhs>::value, "rhs argument for join() has to be a table");

include/sqlpp11/like.h

@@ -37,7 +37,7 @@ namespace sqlpp
 		struct like_t: public boolean::template expression_operators<like_t<Operand, Pattern>>,
 		public alias_operators<like_t<Operand, Pattern>>
 	{
-		using _traits = make_traits<boolean, ::sqlpp::tag::expression, ::sqlpp::tag::named_expression>;
+		using _traits = make_traits<boolean, ::sqlpp::tag::is_expression, ::sqlpp::tag::is_named_expression>;
 		using _recursive_traits = make_recursive_traits<Operand, Pattern>;
 
 		static_assert(is_text_t<Operand>::value, "Operand for like() has to be a text");

include/sqlpp11/limit.h

@@ -54,7 +54,7 @@ namespace sqlpp
 	template<typename Limit>
 		struct limit_t
 		{
-			using _traits = make_traits<no_value_t, ::sqlpp::tag::limit>;
+			using _traits = make_traits<no_value_t, ::sqlpp::tag::is_limit>;
 			using _recursive_traits = make_recursive_traits<Limit>;
 
 			static_assert(is_integral_t<Limit>::value, "limit requires an integral value or integral parameter");
@@ -123,7 +123,7 @@ namespace sqlpp
 	template<typename Database>
 		struct dynamic_limit_t
 		{
-			using _traits = make_traits<no_value_t, ::sqlpp::tag::limit>;
+			using _traits = make_traits<no_value_t, ::sqlpp::tag::is_limit>;
 			using _recursive_traits = make_recursive_traits<>;
 
 			// Data
@@ -172,7 +172,7 @@ namespace sqlpp
 
 	struct no_limit_t
 	{
-		using _traits = make_traits<no_value_t, ::sqlpp::tag::noop>;
+		using _traits = make_traits<no_value_t, ::sqlpp::tag::is_noop>;
 		using _recursive_traits = make_recursive_traits<>;
 
 		// Data

include/sqlpp11/max.h

@@ -35,7 +35,7 @@ namespace sqlpp
 		struct max_t: public value_type_of<Expr>::template expression_operators<max_t<Expr>>,
 		public alias_operators<max_t<Expr>>
 	{
-		using _traits = make_traits<value_type_of<Expr>, ::sqlpp::tag::expression, ::sqlpp::tag::named_expression>;
+		using _traits = make_traits<value_type_of<Expr>, ::sqlpp::tag::is_expression, ::sqlpp::tag::is_named_expression>;
 		using _recursive_traits = make_recursive_traits<Expr>;
 
 		static_assert(is_expression_t<Expr>::value, "max() requires a value expression as argument");

include/sqlpp11/min.h

@@ -35,7 +35,7 @@ namespace sqlpp
 		struct min_t: public value_type_of<Expr>::template expression_operators<min_t<Expr>>,
 		public alias_operators<min_t<Expr>>
 	{
-		using _traits = make_traits<value_type_of<Expr>, ::sqlpp::tag::expression, ::sqlpp::tag::named_expression>;
+		using _traits = make_traits<value_type_of<Expr>, ::sqlpp::tag::is_expression, ::sqlpp::tag::is_named_expression>;
 		using _recursive_traits = make_recursive_traits<Expr>;
 
 		static_assert(is_expression_t<Expr>::value, "min() requires a value expression as argument");

include/sqlpp11/multi_column.h

@@ -76,7 +76,7 @@ namespace sqlpp
 	template<typename AliasProvider, typename... Columns>
 		struct multi_column_alias_t
 		{
-			using _traits = make_traits<no_value_t, tag::alias, tag::multi_column, tag::named_expression>;
+			using _traits = make_traits<no_value_t, tag::is_alias, tag::is_multi_column, tag::is_named_expression>;
 			using _recursive_traits = make_recursive_traits<Columns...>;
 
 			static_assert(detail::all_t<is_named_expression_t<Columns>::value...>::value, "multi_column parameters need to be named expressions");

include/sqlpp11/noop.h

@@ -35,7 +35,7 @@ namespace sqlpp
 {
 	struct noop 
 	{
-		using _traits = make_traits<no_value_t, ::sqlpp::tag::noop>;
+		using _traits = make_traits<no_value_t, ::sqlpp::tag::is_noop>;
 		using _recursive_traits = make_recursive_traits<>;
 
 		struct _name_t {};

include/sqlpp11/null.h

@@ -33,7 +33,7 @@ namespace sqlpp
 {
 	struct null_t
 	{
-		using _traits = make_traits<no_value_t, tag::expression>;
+		using _traits = make_traits<no_value_t, tag::is_expression>;
 		using _recursive_traits = make_recursive_traits<>;
 	};
 

include/sqlpp11/offset.h

@@ -54,7 +54,7 @@ namespace sqlpp
 	template<typename Offset>
 		struct offset_t
 		{
-			using _traits = make_traits<no_value_t, ::sqlpp::tag::offset>;
+			using _traits = make_traits<no_value_t, ::sqlpp::tag::is_offset>;
 			using _recursive_traits = make_recursive_traits<Offset>;
 
 			static_assert(is_integral_t<Offset>::value, "offset requires an integral value or integral parameter");
@@ -123,7 +123,7 @@ namespace sqlpp
 	template<typename Database>
 		struct dynamic_offset_t
 		{
-			using _traits = make_traits<no_value_t, ::sqlpp::tag::offset>;
+			using _traits = make_traits<no_value_t, ::sqlpp::tag::is_offset>;
 			using _recursive_traits = make_recursive_traits<>;
 
 			// Data
@@ -183,7 +183,7 @@ namespace sqlpp
 
 	struct no_offset_t
 	{
-		using _traits = make_traits<no_value_t, ::sqlpp::tag::noop>;
+		using _traits = make_traits<no_value_t, ::sqlpp::tag::is_noop>;
 		using _recursive_traits = make_recursive_traits<>;
 
 		// Data

include/sqlpp11/on.h

@@ -37,7 +37,7 @@ namespace sqlpp
 	template<typename Database, typename... Expr>
 		struct on_t
 		{
-			using _traits = make_traits<no_value_t, tag::on>;
+			using _traits = make_traits<no_value_t, tag::is_on>;
 			using _recursive_traits = make_recursive_traits<Expr...>;
 
 			using _is_dynamic = is_database<Database>;

include/sqlpp11/order_by.h

@@ -59,7 +59,7 @@ namespace sqlpp
 	template<typename Database, typename... Expressions>
 		struct order_by_t
 		{
-			using _traits = make_traits<no_value_t, ::sqlpp::tag::order_by>;
+			using _traits = make_traits<no_value_t, ::sqlpp::tag::is_order_by>;
 			using _recursive_traits = make_recursive_traits<Expressions...>;
 
 			using _is_dynamic = is_database<Database>;
@@ -135,7 +135,7 @@ namespace sqlpp
 	// NO ORDER BY YET
 	struct no_order_by_t
 	{
-		using _traits = make_traits<no_value_t, ::sqlpp::tag::noop>;
+		using _traits = make_traits<no_value_t, ::sqlpp::tag::is_noop>;
 		using _recursive_traits = make_recursive_traits<>;
 
 		// Data

include/sqlpp11/parameter.h

@@ -36,13 +36,15 @@ namespace sqlpp
 	template<typename ValueType, typename NameType>
 		struct parameter_t: public ValueType::template expression_operators<parameter_t<ValueType, NameType>>
 	{
-		using _traits = make_traits<ValueType, tag::parameter, tag::expression>;
+		using _traits = make_traits<ValueType, tag::is_parameter, tag::is_expression>;
 		struct _recursive_traits
 		{
 			using _parameters = std::tuple<parameter_t>;
 			using _provided_tables = detail::type_set<>;
+			using _provided_outer_tables = detail::type_set<>;
 			using _required_tables = detail::type_set<>;
 			using _extra_tables = detail::type_set<>;
+			using _can_be_null = std::true_type;
 		};
 
 		using _instance_t = typename NameType::_name_t::template _member_t<typename ValueType::_parameter_t>;

include/sqlpp11/remove.h

@@ -42,7 +42,7 @@ namespace sqlpp
 	struct remove_name_t {};
 	struct remove_t: public statement_name_t<remove_name_t>
 	{
-		using _traits = make_traits<no_value_t, tag::return_value>;
+		using _traits = make_traits<no_value_t, tag::is_return_value>;
 		struct _name_t {};
 
 		template<typename Policies>

include/sqlpp11/result_row.h

@@ -28,7 +28,7 @@
 #define SQLPP_RESULT_ROW_H
 
 #include <map>
-#include <sqlpp11/field.h>
+#include <sqlpp11/field_spec.h>
 #include <sqlpp11/text.h>
 #include <sqlpp11/detail/column_index_sequence.h>
 
@@ -36,14 +36,14 @@ namespace sqlpp
 {
 	namespace detail
 	{
-		template<typename Db, typename IndexSequence, typename... NamedExprs>
+		template<typename Db, typename IndexSequence, typename... FieldSpecs>
 			struct result_row_impl;
 
-		template<typename Db, std::size_t index, typename NamedExpr>
+		template<typename Db, std::size_t index, typename FieldSpec>
 			struct result_field:
-				public NamedExpr::_name_t::template _member_t<typename value_type_of<NamedExpr>::template _result_entry_t<Db, NamedExpr::_trivial_value_is_null>>
+				public FieldSpec::_name_t::template _member_t<typename value_type_of<FieldSpec>::template _result_field_t<Db, FieldSpec>>
 		{
-			using _field = typename NamedExpr::_name_t::template _member_t<typename value_type_of<NamedExpr>::template _result_entry_t<Db, NamedExpr::_trivial_value_is_null>>;
+			using _field = typename FieldSpec::_name_t::template _member_t<typename value_type_of<FieldSpec>::template _result_field_t<Db, FieldSpec>>;
 
 			result_field() = default;
 
@@ -64,11 +64,11 @@ namespace sqlpp
 				}
 		};
 
-		template<std::size_t index, typename AliasProvider, typename Db, typename... NamedExprs>
+		template<std::size_t index, typename AliasProvider, typename Db, typename... FieldSpecs>
-			struct result_field<Db, index, multi_field_t<AliasProvider, std::tuple<NamedExprs...>>>: 
+			struct result_field<Db, index, multi_field_spec_t<AliasProvider, std::tuple<FieldSpecs...>>>: 
-			public AliasProvider::_name_t::template _member_t<result_row_impl<Db, detail::make_column_index_sequence<index, NamedExprs...>, NamedExprs...>>
+			public AliasProvider::_name_t::template _member_t<result_row_impl<Db, detail::make_column_index_sequence<index, FieldSpecs...>, FieldSpecs...>>
 			{
-				using _multi_field = typename AliasProvider::_name_t::template _member_t<result_row_impl<Db, detail::make_column_index_sequence<index, NamedExprs...>, NamedExprs...>>;
+				using _multi_field = typename AliasProvider::_name_t::template _member_t<result_row_impl<Db, detail::make_column_index_sequence<index, FieldSpecs...>, FieldSpecs...>>;
 
 				result_field() = default;
 
@@ -89,9 +89,9 @@ namespace sqlpp
 					}
 			};
 
-		template<typename Db, std::size_t LastIndex, std::size_t... Is, typename... NamedExprs>
+		template<typename Db, std::size_t LastIndex, std::size_t... Is, typename... FieldSpecs>
-			struct result_row_impl<Db, detail::column_index_sequence<LastIndex, Is...>, NamedExprs...>: 
+			struct result_row_impl<Db, detail::column_index_sequence<LastIndex, Is...>, FieldSpecs...>: 
-			public result_field<Db, Is, NamedExprs>...
+			public result_field<Db, Is, FieldSpecs>...
 			{
 				static constexpr std::size_t _last_index = LastIndex;
 
@@ -100,29 +100,29 @@ namespace sqlpp
 				void _validate()
 				{
 					using swallow = int[];
-					(void) swallow{(result_field<Db, Is, NamedExprs>::_validate(), 0)...};
+					(void) swallow{(result_field<Db, Is, FieldSpecs>::_validate(), 0)...};
 				}
 
 				void _invalidate()
 				{
 					using swallow = int[];
-					(void) swallow{(result_field<Db, Is, NamedExprs>::_invalidate(), 0)...};
+					(void) swallow{(result_field<Db, Is, FieldSpecs>::_invalidate(), 0)...};
 				}
 
 				template<typename Target>
 					void _bind(Target& target)
 					{
 						using swallow = int[];
-						(void) swallow{(result_field<Db, Is, NamedExprs>::_bind(target), 0)...};
+						(void) swallow{(result_field<Db, Is, FieldSpecs>::_bind(target), 0)...};
 					}
 			};
 
 	}
 
-	template<typename Db, typename... NamedExprs>
+	template<typename Db, typename... FieldSpecs>
-		struct result_row_t: public detail::result_row_impl<Db, detail::make_column_index_sequence<0, NamedExprs...>, NamedExprs...>
+		struct result_row_t: public detail::result_row_impl<Db, detail::make_column_index_sequence<0, FieldSpecs...>, FieldSpecs...>
 	{
-		using _impl = detail::result_row_impl<Db, detail::make_column_index_sequence<0, NamedExprs...>, NamedExprs...>;
+		using _impl = detail::result_row_impl<Db, detail::make_column_index_sequence<0, FieldSpecs...>, FieldSpecs...>;
 		bool _is_valid;
 		static constexpr size_t _last_static_index = _impl::_last_index;
 
@@ -178,11 +178,18 @@ namespace sqlpp
 			}
 	};
 
-	template<typename Db, typename... NamedExprs>
+	template<typename Db, typename... FieldSpecs>
-		struct dynamic_result_row_t: public detail::result_row_impl<Db, detail::make_column_index_sequence<0, NamedExprs...>, NamedExprs...>
+		struct dynamic_result_row_t: public detail::result_row_impl<Db, detail::make_column_index_sequence<0, FieldSpecs...>, FieldSpecs...>
 	{
-		using _impl = detail::result_row_impl<Db, detail::make_column_index_sequence<0, NamedExprs...>, NamedExprs...>;
+		using _impl = detail::result_row_impl<Db, detail::make_column_index_sequence<0, FieldSpecs...>, FieldSpecs...>;
-		using _field_type = detail::text::_result_entry_t<Db, false>;
+		struct _field_spec_t
+		{
+			using _traits = make_traits<detail::text, tag::is_noop, tag::can_be_null, tag::null_is_trivial_value>;
+			using _recursive_traits = make_recursive_traits<>;
+
+			struct _name_t {};
+		};
+		using _field_type = detail::text::_result_field_t<Db, _field_spec_t>;
 		static constexpr size_t _last_static_index = _impl::_last_index;
 
 		bool _is_valid;

include/sqlpp11/rhs_is_null.h

@@ -24,48 +24,44 @@
  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-#ifndef SQLPP_FIELD_H
+#ifndef SQLPP_RHS_IS_NULL_H
-#define SQLPP_FIELD_H
+#define SQLPP_RHS_IS_NULL_H
 
-#include <sqlpp11/multi_column.h>
+#include <sqlpp11/tvin.h>
 
 namespace sqlpp
 {
-	template<typename NameType, typename ValueType, bool TrivialValueIsNull>
+	template<typename T, typename Enable = void>
-		struct field_t
+		struct is_trivial_t
 		{
-			using _traits = make_traits<ValueType, tag::noop>;
+			static constexpr bool _(const T&)
-			using _recursive_traits = make_recursive_traits<>;
+			{
-
+				return false;
-			using _name_t = NameType;
+			}
-			static constexpr bool _trivial_value_is_null = TrivialValueIsNull;
 		};
 
-	template<typename AliasProvider, typename FieldTuple>
+	template<typename T>
-		struct multi_field_t
+		struct is_trivial_t<T, typename std::enable_if<std::is_member_function_pointer<decltype(&T::_is_trivial)>::value, void>::type>
 		{
+			static bool _(const T& t)
+			{
+				return t._is_trivial();
+			}
 		};
 
-	namespace detail
+	template<typename T>
-	{
+		bool is_trivial(const T& t)
-		template<typename NamedExpr>
+		{
-			struct make_field_t_impl
+			return is_trivial_t<T>::_(t);
-			{
+		}
-				using type = field_t<typename NamedExpr::_name_t, 
-							value_type_of<NamedExpr>,
-							trivial_value_is_null_t<NamedExpr>::value>;
-			};
-
-		template<typename AliasProvider, typename... NamedExpr>
-			struct make_field_t_impl<multi_column_alias_t<AliasProvider, NamedExpr...>>
-			{
-				using type = multi_field_t<AliasProvider, std::tuple<typename make_field_t_impl<NamedExpr>::type...>>;
-			};
-	}
-
-	template<typename NamedExpr>
-		using make_field_t = typename detail::make_field_t_impl<NamedExpr>::type;
 
+	template<typename Expression>
+		constexpr bool rhs_is_null(const Expression& e)
+		{
+			return (((trivial_value_is_null_t<typename Expression::_lhs_t>::value or is_tvin_t<typename Expression::_rhs_t>::value)
+							and is_trivial(e._rhs))
+						or (std::is_same<typename Expression::_rhs_t, null_t>::value));
+		}
 }
 
 #endif

include/sqlpp11/select_column_list.h

@@ -31,7 +31,7 @@
 #include <sqlpp11/result_row.h>
 #include <sqlpp11/table.h>
 #include <sqlpp11/no_value.h>
-#include <sqlpp11/field.h>
+#include <sqlpp11/field_spec.h>
 #include <sqlpp11/expression_fwd.h>
 #include <sqlpp11/select_pseudo_table.h>
 #include <sqlpp11/named_interpretable.h>
@@ -47,14 +47,14 @@ namespace sqlpp
 		template<typename... Columns>
 			struct select_traits
 			{
-				using _traits = make_traits<no_value_t, tag::select_column_list, tag::return_value>;
+				using _traits = make_traits<no_value_t, tag::is_select_column_list, tag::is_return_value>;
 				struct _name_t {};
 			};
 
 		template<typename Column>
 			struct select_traits<Column>
 			{
-				using _traits = make_traits<value_type_of<Column>, tag::select_column_list, tag::return_value, tag::expression, tag::named_expression>;
+				using _traits = make_traits<value_type_of<Column>, tag::is_select_column_list, tag::is_return_value, tag::is_expression, tag::is_named_expression>;
 				using _name_t = typename Column::_name_t;
 			};
 	}
@@ -245,10 +245,19 @@ namespace sqlpp
 						return static_cast<const _statement_t&>(*this);
 					}
 
+					template<typename Db, typename Column>
+					 struct	_deferred_field_t
+					 {
+						 using type = make_field_spec_t<_statement_t, Column>;
+					 };
+
+					template<typename Db, typename Column>
+						using _field_t = typename _deferred_field_t<Db, Column>::type;
+
 					template<typename Db>
 						using _result_row_t = typename std::conditional<_is_dynamic::value,
-									dynamic_result_row_t<Db, make_field_t<Columns>...>,
+									dynamic_result_row_t<Db, _field_t<Db, Columns>...>,
-									result_row_t<Db, make_field_t<Columns>...>>::type;
+									result_row_t<Db, _field_t<Db, Columns>...>>::type;
 
 					using _dynamic_names_t = typename dynamic_select_column_list<Database>::_names_t;
 
@@ -317,7 +326,7 @@ namespace sqlpp
 
 	struct no_select_column_list_t
 	{
-		using _traits = make_traits<no_value_t, ::sqlpp::tag::noop, ::sqlpp::tag::missing>;
+		using _traits = make_traits<no_value_t, ::sqlpp::tag::is_noop, ::sqlpp::tag::is_missing>;
 		using _recursive_traits = make_recursive_traits<>;
 
 		struct _name_t {};

include/sqlpp11/select_flag_list.h

@@ -59,7 +59,7 @@ namespace sqlpp
 	template<typename Database, typename... Flags>
 		struct select_flag_list_t
 		{
-			using _traits = make_traits<no_value_t, ::sqlpp::tag::select_flag_list>;
+			using _traits = make_traits<no_value_t, ::sqlpp::tag::is_select_flag_list>;
 			using _recursive_traits = make_recursive_traits<Flags...>;
 
 			using _is_dynamic = is_database<Database>;
@@ -134,7 +134,7 @@ namespace sqlpp
 
 	struct no_select_flag_list_t
 	{
-		using _traits = make_traits<no_value_t, ::sqlpp::tag::noop>;
+		using _traits = make_traits<no_value_t, ::sqlpp::tag::is_noop>;
 		using _recursive_traits = make_recursive_traits<>;
 
 		// Data

include/sqlpp11/select_flags.h

@@ -37,7 +37,7 @@ namespace sqlpp
 	// standard select flags
 	struct all_t
 	{
-		using _traits = make_traits<no_value_t, tag::select_flag>;
+		using _traits = make_traits<no_value_t, tag::is_select_flag>;
 		using _recursive_traits = make_recursive_traits<>;
 	};
 	static constexpr all_t all = {};
@@ -54,7 +54,7 @@ namespace sqlpp
 
 	struct distinct_t
 	{
-		using _traits = make_traits<no_value_t, tag::select_flag>;
+		using _traits = make_traits<no_value_t, tag::is_select_flag>;
 		using _recursive_traits = make_recursive_traits<>;
 	};
 	static constexpr distinct_t distinct = {};
@@ -71,7 +71,7 @@ namespace sqlpp
 
 	struct straight_join_t
 	{
-		using _traits = make_traits<no_value_t, tag::select_flag>;
+		using _traits = make_traits<no_value_t, tag::is_select_flag>;
 		using _recursive_traits = make_recursive_traits<>;
 	};
 	static constexpr straight_join_t straight_join = {};

include/sqlpp11/select_pseudo_table.h

@@ -32,19 +32,19 @@
 namespace sqlpp
 {
 	// provide type information for sub-selects that are used as named expressions or tables
-	template<typename ValueType, typename NameType>
+	template<typename Select, typename NamedExpr>
 		struct select_column_spec_t
 		{
-			using _traits = make_traits<ValueType>;
+			using _name_t = typename NamedExpr::_name_t;
-			using _recursive_traits = make_recursive_traits<>;
-			using _value_type = ValueType; // FIXME: column specs probably should use _traits, too
 
-			using _name_t = NameType;
+			static constexpr bool _can_be_null = can_be_null_t<NamedExpr>::value;
-			struct _column_type 
+			static constexpr bool _depends_on_outer_table = detail::make_intersect_set_t<required_tables_of<NamedExpr>, typename Select::_used_outer_tables>::size::value > 0;
-			{
+
-				using _must_not_insert = std::true_type;
+			using _traits = make_traits<value_type_of<NamedExpr>, 
-				using _must_not_update = std::true_type;
+						tag::must_not_insert, 
-			};
+						tag::must_not_update,
+						tag_if<tag::can_be_null, _can_be_null or _depends_on_outer_table>
+							>;
 		};
 
 	template<
@@ -53,9 +53,9 @@ namespace sqlpp
 							 >
 							 struct select_pseudo_table_t: public sqlpp::table_t<select_pseudo_table_t<
 																						 Select,
-																						 NamedExpr...>, select_column_spec_t<value_type_of<NamedExpr>, typename NamedExpr::_name_t>...>
+																						 NamedExpr...>, select_column_spec_t<Select, NamedExpr>...>
 	{
-		using _traits = make_traits<no_value_t, tag::table, tag::pseudo_table>;
+		using _traits = make_traits<no_value_t, tag::is_table, tag::is_pseudo_table>;
 		using _recursive_traits = make_recursive_traits<>;
 
 		select_pseudo_table_t(Select select):

include/sqlpp11/simple_column.h

@@ -37,7 +37,7 @@ namespace sqlpp
 		{
 			Column _column;
 
-			using _traits = make_traits<no_value_t, ::sqlpp::tag::noop>;
+			using _traits = make_traits<no_value_t, ::sqlpp::tag::is_noop>;
 			using _recursive_traits = make_recursive_traits<>;
 		};
 

include/sqlpp11/single_table.h

@@ -57,7 +57,7 @@ namespace sqlpp
 	template<typename Database, typename Table>
 		struct single_table_t
 		{
-			using _traits = make_traits<no_value_t, ::sqlpp::tag::single_table>;
+			using _traits = make_traits<no_value_t, ::sqlpp::tag::is_single_table>;
 			using _recursive_traits = make_recursive_traits<Table>;
 
 			static_assert(is_table_t<Table>::value, "argument has to be a table");
@@ -103,7 +103,7 @@ namespace sqlpp
 	// NO INTO YET
 	struct no_single_table_t
 	{
-		using _traits = make_traits<no_value_t, ::sqlpp::tag::noop>;
+		using _traits = make_traits<no_value_t, ::sqlpp::tag::is_noop>;
 		using _recursive_traits = make_recursive_traits<>;
 
 		// Data

include/sqlpp11/some.h

@@ -35,7 +35,7 @@ namespace sqlpp
 	template<typename Select>
 		struct some_t
 		{
-			using _traits = make_traits<value_type_of<Select>, ::sqlpp::tag::multi_expression>;
+			using _traits = make_traits<value_type_of<Select>, ::sqlpp::tag::is_multi_expression>;
 			using _recursive_traits = make_recursive_traits<Select>;
 
 			struct _name_t

include/sqlpp11/sort_order.h

@@ -41,7 +41,7 @@ namespace sqlpp
 	template<typename Expression, sort_type SortType>
 		struct sort_order_t
 		{
-			using _traits = make_traits<no_value_t,  tag::sort_order, sqlpp::tag::expression>;
+			using _traits = make_traits<no_value_t,  tag::is_sort_order, sqlpp::tag::is_expression>;
 			using _recursive_traits = make_recursive_traits<Expression>;
 
 			Expression _expression;

include/sqlpp11/statement.h

@@ -63,6 +63,7 @@ namespace sqlpp
 
 				using _all_required_tables = detail::make_joined_set_t<required_tables_of<Policies>...>;
 				using _all_provided_tables = detail::make_joined_set_t<provided_tables_of<Policies>...>;
+				using _all_provided_outer_tables = detail::make_joined_set_t<provided_outer_tables_of<Policies>...>;
 				using _all_extra_tables = detail::make_joined_set_t<extra_tables_of<Policies>...>;
 
 				using _known_tables = detail::make_joined_set_t<_all_provided_tables, _all_extra_tables>;
@@ -97,19 +98,21 @@ namespace sqlpp
 					no_value_t // if a required statement part is missing (e.g. columns in a select), then the statement cannot be used as a value
 						>::type;
 
-				using _is_expression = typename std::conditional<
+				using _traits = make_traits<_value_type, tag_if<tag::is_expression, not std::is_same<_value_type, no_value_t>::value>>;
-					std::is_same<_value_type, no_value_t>::value, 
-					std::false_type, 
-					std::true_type>::type;
-
-				using _traits = make_traits<_value_type>;
 
 				struct _recursive_traits
 				{
 					using _required_tables = statement_policies_t::_required_tables;
 					using _provided_tables = detail::type_set<>;
+					using _provided_outer_tables = detail::type_set<>;
 					using _extra_tables = detail::type_set<>;
 					using _parameters = detail::make_parameter_tuple_t<parameters_of<Policies>...>;
+					using _can_be_null = detail::any_t<
+									can_be_null_t<_result_type_provider>::value, 
+									::sqlpp::detail::make_intersect_set_t<
+									  required_tables_of<_result_type_provider>, 
+								    provided_outer_tables_of<statement_policies_t>
+								  >::size::value>;
 				};
 			};
 	}
@@ -125,13 +128,16 @@ namespace sqlpp
 	{
 		using _policies_t = typename detail::statement_policies_t<Db, Policies...>;
 
-		using _traits = make_traits<value_type_of<_policies_t>, ::sqlpp::tag::select, tag::expression_if<typename _policies_t::_is_expression>, tag::named_expression_if<typename _policies_t::_is_expression>>;
+		using _traits = make_traits<value_type_of<_policies_t>,
+					::sqlpp::tag::is_select, 
+					tag_if<tag::is_expression, is_expression_t<_policies_t>::value>, 
+					tag_if<tag::is_named_expression, is_expression_t<_policies_t>::value>,
+					tag::requires_braces>;
 		using _recursive_traits = typename _policies_t::_recursive_traits;
+		using _used_outer_tables = typename _policies_t::_all_provided_outer_tables;
 
 		using _result_type_provider = typename _policies_t::_result_type_provider;
 
-		using _requires_braces = std::true_type;
-
 		using _name_t = typename _result_type_provider::_name_t;
 
 		// Constructors
@@ -192,7 +198,7 @@ namespace sqlpp
 	template<typename NameData>
 		struct statement_name_t
 		{
-			using _traits = make_traits<no_value_t, ::sqlpp::tag::noop>;
+			using _traits = make_traits<no_value_t, ::sqlpp::tag::is_noop>;
 			using _recursive_traits = make_recursive_traits<>;
 
 			// Data

include/sqlpp11/sum.h

@@ -35,7 +35,7 @@ namespace sqlpp
 		struct sum_t: public value_type_of<Expr>::template expression_operators<sum_t<Flag, Expr>>,
 		public alias_operators<sum_t<Flag, Expr>>
 	{
-		using _traits = make_traits<value_type_of<Expr>, ::sqlpp::tag::expression, ::sqlpp::tag::named_expression>;
+		using _traits = make_traits<value_type_of<Expr>, ::sqlpp::tag::is_expression, ::sqlpp::tag::is_named_expression>;
 		using _recursive_traits = make_recursive_traits<Expr>;
 
 		static_assert(is_noop<Flag>::value or std::is_same<sqlpp::distinct_t, Flag>::value, "sum() used with flag other than 'distinct'");

include/sqlpp11/table.h

@@ -42,14 +42,16 @@ namespace sqlpp
 	template<typename Table, typename... ColumnSpec>
 		struct table_t: public table_base_t, public ColumnSpec::_name_t::template _member_t<column_t<Table, ColumnSpec>>...
 	{
-		using _traits = make_traits<no_value_t, tag::table>;
+		using _traits = make_traits<no_value_t, tag::is_table>;
 
 		struct _recursive_traits
 		{
 			using _parameters = std::tuple<>;
 			using _required_tables = detail::type_set<>;
 			using _provided_tables = detail::type_set<Table>;
+			using _provided_outer_tables = detail::type_set<>;
 			using _extra_tables = detail::type_set<>;
+			using _can_be_null = std::false_type;
 		};
 
 		static_assert(sizeof...(ColumnSpec), "at least one column required per table");

include/sqlpp11/table_alias.h

@@ -39,14 +39,16 @@ namespace sqlpp
 		struct table_alias_t: public ColumnSpec::_name_t::template _member_t<column_t<AliasProvider, ColumnSpec>>...
 	{
 		//FIXME: Need to add join functionality
-		using _traits = make_traits<value_type_of<Table>, tag::table, tag::alias, tag::named_expression_if<is_expression_t<Table>>>;
+		using _traits = make_traits<value_type_of<Table>, tag::is_table, tag::is_alias, tag_if<tag::is_named_expression, is_expression_t<Table>::value>>;
 
 		struct _recursive_traits
 		{
 			using _parameters = std::tuple<>;
 			using _required_tables = detail::type_set<>;
 			using _provided_tables = detail::type_set<AliasProvider>;
+			using _provided_outer_tables = detail::type_set<>;
 			using _extra_tables = detail::type_set<>;
+			using _can_be_null = std::false_type;
 		};
 
 		static_assert(required_tables_of<Table>::size::value == 0, "table aliases must not depend on external tables");

include/sqlpp11/text.h

@@ -27,7 +27,6 @@
 #ifndef SQLPP_TEXT_H
 #define SQLPP_TEXT_H
 
-#include <cassert>
 #include <sqlpp11/basic_expression_operators.h>
 #include <sqlpp11/type_traits.h>
 #include <sqlpp11/exception.h>
@@ -41,7 +40,8 @@ namespace sqlpp
 		// text value type
 		struct text
 		{
-			using _tag = ::sqlpp::tag::text;
+			using _traits = make_traits<text, ::sqlpp::tag::is_text, ::sqlpp::tag::is_expression>;
+			using _tag = ::sqlpp::tag::is_text;
 			using _cpp_value_type = std::string;
 
 			struct _parameter_t
@@ -95,10 +95,46 @@ namespace sqlpp
 				bool _is_null;
 			};
 
-			template<typename Db, bool NullIsTrivial = false>
+			template<typename Db, typename FieldSpec>
-				struct _result_entry_t
+				struct _result_field_t;
+
+			// I am SO waiting for concepts lite!
+			template<typename Field, typename Enable = void>
+				struct field_methods_t
 				{
-					_result_entry_t():
+					static constexpr bool _null_is_trivial = true;
+					operator _cpp_value_type() const { return static_cast<const Field&>(*this).value(); }
+				};
+
+			template<typename Db, typename FieldSpec>
+				struct field_methods_t<
+						_result_field_t<Db, FieldSpec>, 
+				    typename std::enable_if<enforce_null_result_treatment_t<Db>::value 
+							and column_spec_can_be_null_t<FieldSpec>::value
+							and not null_is_trivial_value_t<FieldSpec>::value>::type>
+				{
+					static constexpr bool _null_is_trivial = false;
+				};
+
+			template<typename Db, typename FieldSpec>
+				struct _result_field_t: public field_methods_t<_result_field_t<Db, FieldSpec>>
+				{
+					using _field_methods_t = field_methods_t<_result_field_t<Db, FieldSpec>>;
+
+					using _traits = make_traits<integral,
+								tag_if<tag::null_is_trivial_value, _field_methods_t::_null_is_trivial>>;
+
+					struct _recursive_traits
+					{
+						using _parameters = std::tuple<>;
+						using _provided_tables = detail::type_set<>;
+						using _provided_outer_tables = detail::type_set<>;
+						using _required_tables = detail::type_set<>;
+						using _extra_tables = detail::type_set<>;
+						using _can_be_null = column_spec_can_be_null_t<FieldSpec>;
+					};
+
+					_result_field_t():
 						_is_valid(false),
 						_value_ptr(nullptr),
 						_len(0)
@@ -121,35 +157,29 @@ namespace sqlpp
 
 					bool is_null() const
 					{ 
-						if (connector_assert_result_validity_t<Db>::value)
+						if (not _is_valid)
-							assert(_is_valid);
-						else if (not _is_valid)
 							throw exception("accessing is_null in non-existing row");
 						return _value_ptr == nullptr; 
 					}
 
 					_cpp_value_type value() const
 					{
-						const bool null_value = _value_ptr == nullptr and not NullIsTrivial and not connector_null_result_is_trivial_value_t<Db>::value;
+						if (not _is_valid)
-						if (connector_assert_result_validity_t<Db>::value)
+							throw exception("accessing value in non-existing row");
-						{
-							assert(_is_valid);
-							assert(not null_value);
-						}
-						else
-						{
-							if (not _is_valid)
-								throw exception("accessing value in non-existing row");
-							if (null_value)
-								throw exception("accessing value of NULL field");
-						}
-						if (_value_ptr) 
-							return std::string(_value_ptr, _value_ptr + _len);
-						else
-							return "";
-					}
 
-					operator _cpp_value_type() const { return value(); }
+						if (not _value_ptr)
+						{
+							if (enforce_null_result_treatment_t<Db>::value and not null_is_trivial_value_t<FieldSpec>::value)
+							{
+								throw exception("accessing value of NULL field");
+							}
+							else
+							{
+								return "";
+							}
+						}
+						return std::string(_value_ptr, _value_ptr + _len);
+					}
 
 					template<typename Target>
 						void _bind(Target& target, size_t i)
@@ -208,8 +238,8 @@ namespace sqlpp
 				};
 		};
 
-		template<typename Db, bool TrivialIsNull>
+		template<typename Db, typename FieldSpec>
-			inline std::ostream& operator<<(std::ostream& os, const text::_result_entry_t<Db, TrivialIsNull>& e)
+			inline std::ostream& operator<<(std::ostream& os, const text::_result_field_t<Db, FieldSpec>& e)
 			{
 				return os << e.value();
 			}

include/sqlpp11/tvin.h

@@ -37,16 +37,66 @@
 namespace sqlpp
 {
 	template<typename Operand>
-		struct tvin_t
+		struct tvin_arg_t
 		{
-			using _traits = make_traits<value_type_of<Operand>, tag::expression>;
+			using _traits = make_traits<value_type_of<Operand>, tag::is_expression>;
 			using _recursive_traits = make_recursive_traits<Operand>;
 
 			using _operand_t = Operand;
 
-			tvin_t(Operand operand): 
+			tvin_arg_t(_operand_t operand): 
 				_value(operand)
 			{}
+			tvin_arg_t(const tvin_arg_t&) = default;
+			tvin_arg_t(tvin_arg_t&&) = default;
+			tvin_arg_t& operator=(const tvin_arg_t&) = default;
+			tvin_arg_t& operator=(tvin_arg_t&&) = default;
+			~tvin_arg_t() = default;
+
+			_operand_t _value;
+		};
+
+	template<typename Context, typename Operand>
+		struct serializer_t<Context, tvin_arg_t<Operand>>
+		{
+			using T = tvin_arg_t<Operand>;
+
+			static Context& _(const T& t, Context& context)
+			{
+				static_assert(wrong_t<Context, Operand>::value, "tvin may only be used with operators =, == and !=");
+			}
+		};
+
+	template<typename T>
+		struct tvin_t;
+
+	namespace detail
+	{
+	template<typename T>
+		struct allow_tvin_impl
+		{
+			using type = T;
+		};
+	template <typename T>
+		struct allow_tvin_impl<tvin_arg_t<T>>
+		{
+			using type = tvin_t<T>;
+		};
+	}
+	template<typename T>
+		using allow_tvin_t = typename detail::allow_tvin_impl<T>::type;
+
+	template<typename Operand>
+		struct tvin_t
+		{
+			using _traits = make_traits<value_type_of<Operand>, tag::is_expression>;
+			using _recursive_traits = make_recursive_traits<Operand>;
+
+			using _operand_t = Operand;
+
+			tvin_t(tvin_arg_t<Operand> arg): 
+				_value(arg._value)
+			{}
 			tvin_t(const tvin_t&) = default;
 			tvin_t(tvin_t&&) = default;
 			tvin_t& operator=(const tvin_t&) = default;
@@ -58,71 +108,30 @@ namespace sqlpp
 				return _value._is_trivial();
 			}
 
-			Operand _value;
+			_operand_t _value;
 		};
 
+	namespace detail
+	{
+		template<typename T>
+			struct is_tvin_impl
+			{
+				using type = std::false_type;
+			};
+		template<typename T>
+			struct is_tvin_impl<tvin_t<T>>
+			{
+				using type = std::true_type;
+			};
+	}
+	template<typename T>
+		using is_tvin_t = typename detail::is_tvin_impl<T>::type;
+
 	template<typename Context, typename Operand>
 		struct serializer_t<Context, tvin_t<Operand>>
 		{
 			using T = tvin_t<Operand>;
 
-			static void _(const T& t, Context& context)
-			{
-				static_assert(wrong_t<T>::value, "tvin() must not be used with anything but =, ==, != and !");
-			}
-		};
-
-	template<typename Operand>
-		struct maybe_tvin_t
-		{
-			using _traits = make_traits<value_type_of<Operand>, tag::expression>;
-			using _recursive_traits = make_recursive_traits<Operand>;
-
-			static constexpr bool _is_trivial()
-			{
-				return false;
-			}
-
-			maybe_tvin_t(Operand operand): 
-				_value(operand)
-			{}
-			maybe_tvin_t(const maybe_tvin_t&) = default;
-			maybe_tvin_t(maybe_tvin_t&&) = default;
-			maybe_tvin_t& operator=(const maybe_tvin_t&) = default;
-			maybe_tvin_t& operator=(maybe_tvin_t&&) = default;
-			~maybe_tvin_t() = default;
-
-			Operand _value;
-		};
-
-	template<typename Operand>
-		struct maybe_tvin_t<tvin_t<Operand>>
-		{
-			using _traits = make_traits<value_type_of<Operand>, tag::expression>;
-			using _recursive_traits = make_recursive_traits<Operand>;
-
-			bool _is_trivial() const
-			{
-				return _value._is_trivial();
-			};
-
-			maybe_tvin_t(tvin_t<Operand> operand): 
-				_value(operand._value)
-			{}
-			maybe_tvin_t(const maybe_tvin_t&) = default;
-			maybe_tvin_t(maybe_tvin_t&&) = default;
-			maybe_tvin_t& operator=(const maybe_tvin_t&) = default;
-			maybe_tvin_t& operator=(maybe_tvin_t&&) = default;
-			~maybe_tvin_t() = default;
-
-			typename tvin_t<Operand>::_operand_t _value;
-		};
-
-	template<typename Context, typename Operand>
-		struct serializer_t<Context, maybe_tvin_t<Operand>>
-		{
-			using T = maybe_tvin_t<Operand>;
-
 			static Context& _(const T& t, Context& context)
 			{
 				if (t._is_trivial())
@@ -138,7 +147,7 @@ namespace sqlpp
 		};
 
 	template<typename Operand>
-		auto tvin(Operand operand) -> tvin_t<typename wrap_operand<Operand>::type>
+		auto tvin(Operand operand) -> tvin_arg_t<typename wrap_operand<Operand>::type>
 		{
 			using _operand_t = typename wrap_operand<Operand>::type;
 			static_assert(std::is_same<_operand_t, text_operand>::value

include/sqlpp11/type_traits.h

@@ -32,129 +32,109 @@
 
 namespace sqlpp
 {
-#define SQLPP_IS_VALUE_TRAIT_GENERATOR(name) \
+	namespace detail
+	{
+		template<typename T, typename Enable = void>
+		struct can_be_null_impl { using type = std::false_type; };
+		template<typename T>
+		struct can_be_null_impl<T, typename std::enable_if<T::_recursive_traits::_can_be_null::value>::type> { using type = std::true_type; };
+	}
+	template<typename T>
+	using can_be_null_t = typename detail::can_be_null_impl<T>::type;
+
+	namespace tag\
+	{\
+		struct can_be_null{};\
+	};\
 	namespace detail\
 	{\
 		template<typename T, typename Enable = void>\
-		struct is_##name##_impl: std::false_type {};\
+		struct column_spec_can_be_null_impl { using type = std::false_type; };\
 		template<typename T>\
-		struct is_##name##_impl<T, typename std::enable_if<std::is_same<typename T::_value_type::_is_##name, std::true_type>::value>::type>: std::true_type {};\
+		struct column_spec_can_be_null_impl<T, typename std::enable_if<detail::is_element_of<tag::can_be_null, typename T::_traits::_tags>::value>::type> { using type = std::true_type; };\
 	}\
+	template<typename T>\
+	using column_spec_can_be_null_t = typename detail::column_spec_can_be_null_impl<T>::type;
+
+#define SQLPP_VALUE_TRAIT_GENERATOR(name) \
 	namespace tag\
 	{\
 		struct name{};\
 	};\
-	template<typename T>\
-	using is_##name##_t = detail::is_element_of<tag::name, typename T::_traits::_tags>;
-
-#define SQLPP_IS_COLUMN_TRAIT_GENERATOR(name) \
 	namespace detail\
 	{\
 		template<typename T, typename Enable = void>\
 		struct name##_impl { using type = std::false_type; };\
 		template<typename T>\
-		struct name##_impl<T, typename std::enable_if<std::is_same<typename T::_column_type::_##name, std::true_type>::value>::type> { using type = std::true_type; };\
+		struct name##_impl<T, typename std::enable_if<detail::is_element_of<tag::name, typename T::_traits::_tags>::value>::type> { using type = std::true_type; };\
 	}\
 	template<typename T>\
 	using name##_t = typename detail::name##_impl<T>::type;
 
-#define SQLPP_TYPE_TRAIT_GENERATOR(name) \
+	SQLPP_VALUE_TRAIT_GENERATOR(is_boolean);
-	namespace detail\
+	SQLPP_VALUE_TRAIT_GENERATOR(is_integral);
-	{\
+	SQLPP_VALUE_TRAIT_GENERATOR(is_floating_point);
-		template<typename T, typename Enable = void>\
-		struct name##_impl: std::false_type {};\
-		template<typename T>\
-		struct name##_impl<T, typename std::enable_if<std::is_same<typename T::_##name, std::true_type>::value>::type>: std::true_type {};\
-	}\
-	template<typename T>\
-	struct name##_t: detail::name##_impl<T> {};
-
-#define SQLPP_CONNECTOR_TRAIT_GENERATOR(name) \
-	namespace detail\
-	{\
-		template<typename T, typename Enable = void>\
-		struct connector_##name##_impl: std::false_type {};\
-		template<typename T>\
-		struct connector_##name##_impl<T, typename std::enable_if<std::is_same<typename T::_tags::_##name, std::true_type>::value>::type>: std::true_type {};\
-	}\
-	template<typename T>\
-	struct connector_##name##_t: detail::connector_##name##_impl<T> {};
-
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(boolean);
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(integral);
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(floating_point);
 	template<typename T>
 		using is_numeric_t = detail::any_t<
-		detail::is_element_of<tag::integral, typename T::_traits::_tags>::value,
+		detail::is_element_of<tag::is_integral, typename T::_traits::_tags>::value,
-		detail::is_element_of<tag::floating_point, typename T::_traits::_tags>::value>;
+		detail::is_element_of<tag::is_floating_point, typename T::_traits::_tags>::value>;
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(text);
+	SQLPP_VALUE_TRAIT_GENERATOR(is_text);
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(wrapped_value);
+	SQLPP_VALUE_TRAIT_GENERATOR(is_wrapped_value);
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(expression);
+	SQLPP_VALUE_TRAIT_GENERATOR(is_expression);
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(named_expression);
+	SQLPP_VALUE_TRAIT_GENERATOR(is_named_expression);
-	namespace tag
+	SQLPP_VALUE_TRAIT_GENERATOR(is_multi_expression);
-	{
+	SQLPP_VALUE_TRAIT_GENERATOR(is_alias);
-		template<typename C>
+	SQLPP_VALUE_TRAIT_GENERATOR(is_select_flag);
-			using named_expression_if = typename std::conditional<C::value, tag::named_expression, void>::type;
-	}
-	namespace tag
-	{
-		template<typename C>
-			using expression_if = typename std::conditional<C::value, tag::expression, void>::type;
-	}
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(multi_expression);
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(alias);
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(select_flag);
 
-	SQLPP_IS_COLUMN_TRAIT_GENERATOR(must_not_insert);
+	SQLPP_VALUE_TRAIT_GENERATOR(must_not_insert);
-	SQLPP_IS_COLUMN_TRAIT_GENERATOR(must_not_update);
+	SQLPP_VALUE_TRAIT_GENERATOR(must_not_update);
-	SQLPP_IS_COLUMN_TRAIT_GENERATOR(require_insert);
+	SQLPP_VALUE_TRAIT_GENERATOR(require_insert);
-	SQLPP_IS_COLUMN_TRAIT_GENERATOR(can_be_null);
+	SQLPP_VALUE_TRAIT_GENERATOR(trivial_value_is_null);
-	SQLPP_IS_COLUMN_TRAIT_GENERATOR(trivial_value_is_null);
+	SQLPP_VALUE_TRAIT_GENERATOR(null_is_trivial_value);
 
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(noop);
+	SQLPP_VALUE_TRAIT_GENERATOR(is_noop);
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(missing);
+	SQLPP_VALUE_TRAIT_GENERATOR(is_missing);
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(return_value);
+	SQLPP_VALUE_TRAIT_GENERATOR(is_return_value);
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(table);
+	SQLPP_VALUE_TRAIT_GENERATOR(is_table);
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(join);
+	SQLPP_VALUE_TRAIT_GENERATOR(is_join);
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(pseudo_table);
+	SQLPP_VALUE_TRAIT_GENERATOR(is_pseudo_table);
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(column);
+	SQLPP_VALUE_TRAIT_GENERATOR(is_column);
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(select);
+	SQLPP_VALUE_TRAIT_GENERATOR(is_select);
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(select_flag_list);
+	SQLPP_VALUE_TRAIT_GENERATOR(is_select_flag_list);
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(select_column_list);
+	SQLPP_VALUE_TRAIT_GENERATOR(is_select_column_list);
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(from);
+	SQLPP_VALUE_TRAIT_GENERATOR(is_from);
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(single_table);
+	SQLPP_VALUE_TRAIT_GENERATOR(is_single_table);
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(into);
+	SQLPP_VALUE_TRAIT_GENERATOR(is_into);
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(extra_tables);
+	SQLPP_VALUE_TRAIT_GENERATOR(is_extra_tables);
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(on);
+	SQLPP_VALUE_TRAIT_GENERATOR(is_on);
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(where);
+	SQLPP_VALUE_TRAIT_GENERATOR(is_where);
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(group_by);
+	SQLPP_VALUE_TRAIT_GENERATOR(is_group_by);
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(having);
+	SQLPP_VALUE_TRAIT_GENERATOR(is_having);
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(order_by);
+	SQLPP_VALUE_TRAIT_GENERATOR(is_order_by);
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(limit);
+	SQLPP_VALUE_TRAIT_GENERATOR(is_limit);
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(offset);
+	SQLPP_VALUE_TRAIT_GENERATOR(is_offset);
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(using_);
+	SQLPP_VALUE_TRAIT_GENERATOR(is_using_);
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(column_list);
+	SQLPP_VALUE_TRAIT_GENERATOR(is_column_list);
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(multi_column);
+	SQLPP_VALUE_TRAIT_GENERATOR(is_multi_column);
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(value_list);
+	SQLPP_VALUE_TRAIT_GENERATOR(is_value_list);
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(assignment);
+	SQLPP_VALUE_TRAIT_GENERATOR(is_assignment);
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(update_list);
+	SQLPP_VALUE_TRAIT_GENERATOR(is_update_list);
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(insert_list);
+	SQLPP_VALUE_TRAIT_GENERATOR(is_insert_list);
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(insert_value);
+	SQLPP_VALUE_TRAIT_GENERATOR(is_insert_value);
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(insert_value_list);
+	SQLPP_VALUE_TRAIT_GENERATOR(is_insert_value_list);
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(sort_order);
+	SQLPP_VALUE_TRAIT_GENERATOR(is_sort_order);
-	SQLPP_IS_VALUE_TRAIT_GENERATOR(parameter);
+	SQLPP_VALUE_TRAIT_GENERATOR(is_parameter);
 
-	SQLPP_TYPE_TRAIT_GENERATOR(requires_braces);
+	SQLPP_VALUE_TRAIT_GENERATOR(requires_braces);
 
-	SQLPP_CONNECTOR_TRAIT_GENERATOR(null_result_is_trivial_value);
+	SQLPP_VALUE_TRAIT_GENERATOR(enforce_null_result_treatment);
-	SQLPP_CONNECTOR_TRAIT_GENERATOR(assert_result_validity);
+
+	template<typename Tag, bool Condition>
+		using tag_if = typename std::conditional<Condition, Tag, void>::type;
 
 	template<typename Database>
 		using is_database = typename std::conditional<std::is_same<Database, void>::value, std::false_type, std::true_type>::type;
 
-	template<typename T, template<typename> class IsTag>
-		using copy_type_trait = typename std::conditional<IsTag<T>::value, std::true_type, std::false_type>::type;
-
 	namespace detail
 	{
 		template<typename T>
@@ -175,6 +155,12 @@ namespace sqlpp
 				using type = typename T::_recursive_traits::_provided_tables;
 			};
 
+		template<typename T>
+			struct provided_outer_table_of_impl
+			{
+				using type = typename T::_recursive_traits::_provided_outer_tables;
+			};
+
 		template<typename T>
 			struct extra_table_of_impl
 			{
@@ -211,6 +197,9 @@ namespace sqlpp
 	template<typename T>
 		using provided_tables_of = typename detail::provided_table_of_impl<T>::type;
 
+	template<typename T>
+		using provided_outer_tables_of = typename detail::provided_outer_table_of_impl<T>::type;
+
 	template<typename T>
 		using extra_tables_of = typename detail::extra_table_of_impl<T>::type;
 
@@ -231,8 +220,10 @@ namespace sqlpp
 		{
 			using _required_tables = detail::make_joined_set_t<required_tables_of<Arguments>...>;
 			using _provided_tables = detail::make_joined_set_t<provided_tables_of<Arguments>...>;
+			using _provided_outer_tables = detail::make_joined_set_t<provided_outer_tables_of<Arguments>...>;
 			using _extra_tables = detail::make_joined_set_t<extra_tables_of<Arguments>...>;
 			using _parameters = detail::make_parameter_tuple_t<parameters_of<Arguments>...>;
+			using _can_be_null = detail::any_t<can_be_null_t<Arguments>::value...>;
 		};
 
 }

include/sqlpp11/update.h

@@ -43,7 +43,7 @@ namespace sqlpp
 
 	struct update_t: public statement_name_t<update_name_t>
 	{
-		using _traits = make_traits<no_value_t, tag::return_value>;
+		using _traits = make_traits<no_value_t, tag::is_return_value>;
 		struct _name_t {};
 
 		template<typename Policies>

include/sqlpp11/update_list.h

@@ -56,7 +56,7 @@ namespace sqlpp
 	template<typename Database, typename... Assignments>
 		struct update_list_t
 		{
-			using _traits = make_traits<no_value_t, ::sqlpp::tag::update_list>;
+			using _traits = make_traits<no_value_t, ::sqlpp::tag::is_update_list>;
 			using _recursive_traits = make_recursive_traits<Assignments...>;
 			using _is_dynamic = is_database<Database>;
 
@@ -78,9 +78,9 @@ namespace sqlpp
 						{
 							static_assert(_is_dynamic::value, "add must not be called for static from()");
 							static_assert(is_assignment_t<Assignment>::value, "invalid assignment argument in add()");
-							using _assigned_columns = detail::make_type_set_t<typename Assignments::_column_t...>;
+							using _assigned_columns = detail::make_type_set_t<typename Assignments::_lhs_t...>;
-							static_assert(not detail::is_element_of<typename Assignment::_column_t, _assigned_columns>::value, "Must not assign value to column twice");
+							static_assert(not detail::is_element_of<typename Assignment::_lhs_t, _assigned_columns>::value, "Must not assign value to column twice");
-							static_assert(sqlpp::detail::not_t<must_not_update_t, typename Assignment::_column_t>::value, "add() argument must not be updated");
+							static_assert(sqlpp::detail::not_t<must_not_update_t, typename Assignment::_lhs_t>::value, "add() argument must not be updated");
 							static_assert(TableCheckRequired::value or Policies::template _no_unknown_tables<Assignment>::value, "assignment uses tables unknown to this statement in add()");
 
 							using ok = ::sqlpp::detail::all_t<
@@ -130,7 +130,7 @@ namespace sqlpp
 
 	struct no_update_list_t
 	{
-		using _traits = make_traits<no_value_t, ::sqlpp::tag::where>;
+		using _traits = make_traits<no_value_t, ::sqlpp::tag::is_where>;
 		using _recursive_traits = make_recursive_traits<>;
 
 		// Data
@@ -192,9 +192,9 @@ namespace sqlpp
 					{
 						static_assert(not ::sqlpp::detail::has_duplicates<Assignments...>::value, "at least one duplicate argument detected in set()");
 						static_assert(::sqlpp::detail::all_t<is_assignment_t<Assignments>::value...>::value, "at least one argument is not an assignment in set()");
-						static_assert(::sqlpp::detail::none_t<must_not_update_t<typename Assignments::_column_t>::value...>::value, "at least one assignment is prohibited by its column definition in set()");
+						static_assert(::sqlpp::detail::none_t<must_not_update_t<typename Assignments::_lhs_t>::value...>::value, "at least one assignment is prohibited by its column definition in set()");
 
-						using _column_required_tables = typename ::sqlpp::detail::make_joined_set<required_tables_of<typename Assignments::_column_t>...>::type;
+						using _column_required_tables = ::sqlpp::detail::make_joined_set_t<required_tables_of<typename Assignments::_lhs_t>...>;
 						static_assert(sizeof...(Assignments) ? (_column_required_tables::size::value == 1) : true, "set() contains assignments for columns from more than one table");
 
 						return { *static_cast<typename Policies::_statement_t*>(this), update_list_data_t<Database, Assignments...>{assignments...} };

include/sqlpp11/using.h

@@ -57,7 +57,7 @@ namespace sqlpp
 	template<typename Database, typename... Tables>
 		struct using_t
 		{
-			using _traits = make_traits<no_value_t, ::sqlpp::tag::using_>;
+			using _traits = make_traits<no_value_t, ::sqlpp::tag::is_using_>;
 			using _recursive_traits = make_recursive_traits<Tables...>;
 
 			using _is_dynamic = is_database<Database>;
@@ -128,7 +128,7 @@ namespace sqlpp
 	// NO USING YET
 	struct no_using_t
 	{
-		using _traits = make_traits<no_value_t, ::sqlpp::tag::where>;
+		using _traits = make_traits<no_value_t, ::sqlpp::tag::is_where>;
 		using _recursive_traits = make_recursive_traits<>;
 
 		// Data

include/sqlpp11/verbatim_table.h

@@ -42,13 +42,17 @@ namespace sqlpp
 					{
 					};
 			};
-			using _value_type = no_value_t;
+			using _traits = make_traits<no_value_t>;
-			struct _column_type {};
 		};
 	}
 
 	struct verbatim_table_t: public sqlpp::table_t<verbatim_table_t, detail::unusable_pseudo_column_t>
 	{
+		struct _recursive_traits: public sqlpp::table_t<verbatim_table_t, detail::unusable_pseudo_column_t>::_recursive_traits
+		{
+			using _provided_outer_tables = detail::type_set<verbatim_table_t>;
+		};
+
 		struct _name_t {};
 
 		verbatim_table_t(std::string representation):

include/sqlpp11/where.h

@@ -58,7 +58,7 @@ namespace sqlpp
 	template<typename Database, typename... Expressions>
 		struct where_t
 		{
-			using _traits = make_traits<no_value_t, ::sqlpp::tag::where>;
+			using _traits = make_traits<no_value_t, ::sqlpp::tag::is_where>;
 			using _recursive_traits = make_recursive_traits<Expressions...>;
 
 			using _is_dynamic = is_database<Database>;
@@ -141,7 +141,7 @@ namespace sqlpp
 	template<>
 		struct where_t<void, bool>
 		{
-			using _traits = make_traits<no_value_t, ::sqlpp::tag::where>;
+			using _traits = make_traits<no_value_t, ::sqlpp::tag::is_where>;
 			using _recursive_traits = make_recursive_traits<>;
 
 			// Data
@@ -184,7 +184,7 @@ namespace sqlpp
 	template<bool Required>
 		struct no_where_t
 		{
-			using _traits = make_traits<no_value_t, ::sqlpp::tag::where>;
+			using _traits = make_traits<no_value_t, ::sqlpp::tag::is_where>;
 			using _recursive_traits = make_recursive_traits<>;
 
 			// Data

include/sqlpp11/wrap_operand.h

@@ -43,7 +43,7 @@ namespace sqlpp
 
 	struct boolean_operand
 	{
-		using _traits = make_traits<::sqlpp::detail::boolean, ::sqlpp::tag::expression, ::sqlpp::tag::wrapped_value>;
+		using _traits = make_traits<::sqlpp::detail::boolean, ::sqlpp::tag::is_expression, ::sqlpp::tag::is_wrapped_value>;
 		using _recursive_traits = make_recursive_traits<>;
 
 		using _value_t = bool;
@@ -81,7 +81,7 @@ namespace sqlpp
 
 	struct integral_operand
 	{
-		using _traits = make_traits<::sqlpp::detail::integral, ::sqlpp::tag::expression, ::sqlpp::tag::wrapped_value>;
+		using _traits = make_traits<::sqlpp::detail::integral, ::sqlpp::tag::is_expression, ::sqlpp::tag::is_wrapped_value>;
 		using _recursive_traits = make_recursive_traits<>;
 
 		using _value_t = int64_t;
@@ -120,7 +120,7 @@ namespace sqlpp
 
 	struct floating_point_operand
 	{
-		using _traits = make_traits<::sqlpp::detail::floating_point, ::sqlpp::tag::expression, ::sqlpp::tag::wrapped_value>;
+		using _traits = make_traits<::sqlpp::detail::floating_point, ::sqlpp::tag::is_expression, ::sqlpp::tag::is_wrapped_value>;
 		using _recursive_traits = make_recursive_traits<>;
 
 		using _value_t = double;
@@ -158,7 +158,7 @@ namespace sqlpp
 
 	struct text_operand
 	{
-		using _traits = make_traits<::sqlpp::detail::text, ::sqlpp::tag::expression, ::sqlpp::tag::wrapped_value>;
+		using _traits = make_traits<::sqlpp::detail::text, ::sqlpp::tag::is_expression, ::sqlpp::tag::is_wrapped_value>;
 		using _recursive_traits = make_recursive_traits<>;
 
 		using _value_t = std::string;

scripts/ddl2cpp

@@ -155,22 +155,20 @@ for tableCreation in tableCreations:
         print('          };', file=header)
         print('      };', file=header)
         #print(sqlColumnType)
-        print('      using _value_type = ' + NAMESPACE + '::' + types[sqlColumnType] + ';', file=header)
+        traitslist = [NAMESPACE + '::' + types[sqlColumnType]];
-        print('      struct _column_type', file=header)
-        print('      {', file=header)
         requireInsert = True
         if column.hasAutoValue:
-            print('        using _must_not_insert = std::true_type;', file=header)
+            traitslist.append(NAMESPACE + '::tag::must_not_insert');
-            print('        using _must_not_update = std::true_type;', file=header)
+            traitslist.append(NAMESPACE + '::tag::must_not_update');
             requireInsert = False
         if not column.notNull:
-            print('        using _can_be_null = std::true_type;', file=header)
+            traitslist.append(NAMESPACE + '::tag::can_be_null');
             requireInsert = False
         if column.hasDefaultValue:
             requireInsert = False
         if requireInsert:
-            print('        using _require_insert = std::true_type;', file=header)
+            traitslist.append(NAMESPACE + '::tag::require_insert');
-        print('      };', file=header)
+        print('      using _traits = ' + NAMESPACE + '::make_traits<' + ', '.join(traitslist) + '>;', file=header)
         print('    };', file=header)
     print('  }', file=header)
     print('', file=header)

tests/CMakeLists.txt

@@ -15,6 +15,7 @@ build_and_run(SelectTypeTest)
 build_and_run(FunctionTest)
 build_and_run(PreparedTest)
 build_and_run(Minimalistic)
+build_and_run(ResultTest)
 
 # if you want to use the generator, you can do something like this:
 #find_package(PythonInterp REQUIRED)

tests/InterpretTest.cpp

@@ -36,7 +36,6 @@
 
 MockDb db = {};
 MockDb::_serializer_context_t printer;
-SQLPP_ALIAS_PROVIDER(kaesekuchen);
 
 int main()
 {
@@ -125,6 +124,10 @@ int main()
 
 	// join
 	serialize(t.inner_join(t.as(t.alpha)).on(t.beta == t.as(t.alpha).beta), printer).str();
+	{
+		auto inner = t.inner_join(t.as(t.alpha)).on(t.beta == t.as(t.alpha).beta);
+		serialize(select(t.alpha).from(inner), printer).str();
+	}
 
 	// multi_column
 	serialize(multi_column(t.alpha, (t.beta + "cake").as(t.gamma)).as(t.alpha), printer).str();

tests/MockDb.h

@@ -30,8 +30,13 @@
 #include <sqlpp11/serializer_context.h>
 #include <sqlpp11/connection.h>
 
-struct MockDb: public sqlpp::connection
+template<bool enforceNullResultTreatment>
+struct MockDbT: public sqlpp::connection
 {
+	using _traits = ::sqlpp::make_traits<::sqlpp::no_value_t, 
+						::sqlpp::tag_if<::sqlpp::tag::enforce_null_result_treatment, enforceNullResultTreatment>
+					>;
+
 	struct _serializer_context_t
 	{
 		std::ostringstream _os;
@@ -156,5 +161,8 @@ struct MockDb: public sqlpp::connection
 
 };
 
+using MockDb = MockDbT<false>;
+using EnforceDb = MockDbT<true>;
+
 #endif
 

tests/Sample.h

@@ -21,11 +21,7 @@ namespace test
             const T& operator()() const { return delta; }
           };
       };
-      using _value_type = sqlpp::varchar;
+      using _traits = sqlpp::make_traits<sqlpp::varchar, sqlpp::tag::can_be_null>;
-      struct _column_type
-      {
-        using _can_be_null = std::true_type;
-      };
     };
     struct Epsilon
     {
@@ -40,11 +36,7 @@ namespace test
             const T& operator()() const { return epsilon; }
           };
       };
-      using _value_type = sqlpp::bigint;
+      using _traits = sqlpp::make_traits<sqlpp::bigint, sqlpp::tag::can_be_null>;
-      struct _column_type
-      {
-        using _can_be_null = std::true_type;
-      };
     };
     struct Omega
     {
@@ -59,11 +51,7 @@ namespace test
             const T& operator()() const { return omega; }
           };
       };
-      using _value_type = sqlpp::floating_point;
+      using _traits = sqlpp::make_traits<sqlpp::floating_point, sqlpp::tag::can_be_null>;
-      struct _column_type
-      {
-        using _can_be_null = std::true_type;
-      };
     };
   }
 
@@ -100,13 +88,7 @@ namespace test
             const T& operator()() const { return alpha; }
           };
       };
-      using _value_type = sqlpp::bigint;
+      using _traits = sqlpp::make_traits<sqlpp::bigint, sqlpp::tag::must_not_insert, sqlpp::tag::must_not_update, sqlpp::tag::can_be_null>;
-      struct _column_type
-      {
-        using _must_not_insert = std::true_type;
-        using _must_not_update = std::true_type;
-        using _can_be_null = std::true_type;
-      };
     };
     struct Beta
     {
@@ -121,11 +103,7 @@ namespace test
             const T& operator()() const { return beta; }
           };
       };
-      using _value_type = sqlpp::varchar;
+      using _traits = sqlpp::make_traits<sqlpp::varchar, sqlpp::tag::can_be_null>;
-      struct _column_type
-      {
-        using _can_be_null = std::true_type;
-      };
     };
     struct Gamma
     {
@@ -140,11 +118,7 @@ namespace test
             const T& operator()() const { return gamma; }
           };
       };
-      using _value_type = sqlpp::boolean;
+      using _traits = sqlpp::make_traits<sqlpp::boolean, sqlpp::tag::require_insert>;
-      struct _column_type
-      {
-        using _require_insert = std::true_type;
-      };
     };
     struct Delta
     {
@@ -159,11 +133,7 @@ namespace test
             const T& operator()() const { return delta; }
           };
       };
-      using _value_type = sqlpp::integer;
+      using _traits = sqlpp::make_traits<sqlpp::integer, sqlpp::tag::can_be_null>;
-      struct _column_type
-      {
-        using _can_be_null = std::true_type;
-      };
     };
   }
 

tests/SelectTest.cpp

@@ -36,14 +36,6 @@
 MockDb db = {};
 MockDb::_serializer_context_t printer;
 
-namespace alias
-{
-	SQLPP_ALIAS_PROVIDER(a);
-	SQLPP_ALIAS_PROVIDER(b);
-	SQLPP_ALIAS_PROVIDER(left);
-	SQLPP_ALIAS_PROVIDER(right);
-}
-
 int main()
 {
 	test::TabFoo f;