diff --git a/include/sqlpp11/alias.h b/include/sqlpp11/alias.h
index f9d65fc4..7b0b4883 100644
--- a/include/sqlpp11/alias.h
+++ b/include/sqlpp11/alias.h
@@ -33,6 +33,9 @@ namespace sqlpp
 	template<typename Expression, typename AliasProvider>
 		struct expression_alias_t
 		{
+			static_assert(is_expression_t<Expression>::value, "invalid argument for an expression alias");
+			static_assert(not is_alias_t<Expression>::value, "cannot create an alias of an alias");
+
 			struct _value_type: Expression::_value_type
 			{
 				using _is_expression = std::false_type;
diff --git a/include/sqlpp11/any.h b/include/sqlpp11/any.h
index 6bacf054..fbe4a90a 100644
--- a/include/sqlpp11/any.h
+++ b/include/sqlpp11/any.h
@@ -35,14 +35,15 @@ namespace sqlpp
 	namespace vendor
 	{
 		template<typename Select>
-		struct any_t: public boolean::template operators<any_t<Select>>
+		struct any_t
 		{
 			static_assert(is_select_t<Select>::value, "any() requires a single column select expression as argument");
 			static_assert(is_value_t<Select>::value, "any() requires a single column select expression as argument");
 
 			struct _value_type: public Select::_value_type::_base_value_type
 			{
-				using _is_multi_expression = std::true_type; // must not be named
+				using _is_expression = std::false_type;
+				using _is_multi_expression = std::true_type; // must not be named or used with +,-,*,/, etc
 			};
 
 			struct _name_t
@@ -90,8 +91,9 @@ namespace sqlpp
 			};
 
 		template<typename T>
-			auto any(T t) -> typename vendor::any_t<typename operand_t<T, is_select_t>::type>
+			auto any(T t) -> typename vendor::any_t<vendor::wrap_operand_t<T>>
 			{
+				static_assert(is_select_t<vendor::wrap_operand_t<T>>::value, "any() requires a select expression as argument");
 				return { t };
 			}
 
diff --git a/include/sqlpp11/avg.h b/include/sqlpp11/avg.h
index fed9efcb..b174b5b9 100644
--- a/include/sqlpp11/avg.h
+++ b/include/sqlpp11/avg.h
@@ -34,7 +34,7 @@ namespace sqlpp
 	namespace vendor
 	{
 		template<typename Flag, typename Expr>
-		struct avg_t: public floating_point::template operators<avg_t<Flag, Expr>>
+		struct avg_t: public floating_point::template expression_operators<avg_t<Flag, Expr>>
 		{
 			static_assert(is_noop<Flag>::value or std::is_same<sqlpp::distinct_t, Flag>::value, "avg() used with flag other than 'distinct'");
 			static_assert(is_numeric_t<Expr>::value, "avg() requires a value expression as argument");
@@ -93,14 +93,16 @@ namespace sqlpp
 	}
 
 	template<typename T>
-		auto avg(T t) -> typename vendor::avg_t<vendor::noop, typename operand_t<T, is_value_t>::type>
+		auto avg(T t) -> typename vendor::avg_t<vendor::noop, vendor::wrap_operand_t<T>>
 		{
+			static_assert(is_numeric_t<vendor::wrap_operand_t<T>>::value, "avg() requires a value expression as argument");
 			return { t };
 		}
 
 	template<typename T>
-		auto avg(const sqlpp::distinct_t&, T t) -> typename vendor::avg_t<sqlpp::distinct_t, typename operand_t<T, is_value_t>::type>
+		auto avg(const sqlpp::distinct_t&, T t) -> typename vendor::avg_t<sqlpp::distinct_t, vendor::wrap_operand_t<T>>
 		{
+			static_assert(is_numeric_t<vendor::wrap_operand_t<T>>::value, "avg() requires a value expression as argument");
 			return { t };
 		}
 
diff --git a/include/sqlpp11/basic_expression_operators.h b/include/sqlpp11/basic_expression_operators.h
new file mode 100644
index 00000000..72954b38
--- /dev/null
+++ b/include/sqlpp11/basic_expression_operators.h
@@ -0,0 +1,156 @@
+/*
+ * Copyright (c) 2013, 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_DETAIL_BASIC_EXPRESSION_OPERATORS_H
+#define SQLPP_DETAIL_BASIC_EXPRESSION_OPERATORS_H
+
+#include <sqlpp11/alias.h>
+#include <sqlpp11/sort_order.h>
+#include <sqlpp11/vendor/expression_fwd.h>
+#include <sqlpp11/vendor/in_fwd.h>
+#include <sqlpp11/vendor/is_null_fwd.h>
+#include <sqlpp11/detail/logic.h>
+
+namespace sqlpp
+{
+
+	namespace detail
+	{
+		struct boolean;
+	}
+
+	// basic operators
+	template<typename Base, template<typename> class IsCorrectValueType>
+		struct basic_expression_operators
+		{
+			template<typename T>
+				struct _is_valid_comparison_operand
+			{
+				static constexpr bool value = 
+					(is_expression_t<T>::value // expressions are OK
+					or is_multi_expression_t<T>::value) // multi-expressions like ANY are OK for comparisons, too
+					and IsCorrectValueType<T>::value // the correct value type is required, of course
+					;
+			};
+
+			template<typename T>
+				vendor::equal_to_t<Base, vendor::wrap_operand_t<T>> operator==(T t) const
+				{
+					using rhs = vendor::wrap_operand_t<T>;
+					static_assert(_is_valid_comparison_operand<rhs>::value, "invalid rhs operand in comparison");
+
+					return { *static_cast<const Base*>(this), rhs{t} };
+				}
+
+			template<typename T>
+				vendor::not_equal_to_t<Base, vendor::wrap_operand_t<T>> operator!=(T t) const
+				{
+					using rhs = vendor::wrap_operand_t<T>;
+					static_assert(_is_valid_comparison_operand<rhs>::value, "invalid rhs operand in comparison");
+
+					return { *static_cast<const Base*>(this), rhs{t} };
+				}
+
+			template<typename T>
+				vendor::less_than_t<Base, vendor::wrap_operand_t<T>> operator<(T t) const
+				{
+					using rhs = vendor::wrap_operand_t<T>;
+					static_assert(_is_valid_comparison_operand<rhs>::value, "invalid rhs operand in comparison");
+
+					return { *static_cast<const Base*>(this), rhs{t} };
+				}
+
+			template<typename T>
+				vendor::less_equal_t<Base, vendor::wrap_operand_t<T>> operator<=(T t) const
+				{
+					using rhs = vendor::wrap_operand_t<T>;
+					static_assert(_is_valid_comparison_operand<rhs>::value, "invalid rhs operand in comparison");
+
+					return { *static_cast<const Base*>(this), rhs{t} };
+				}
+
+			template<typename T>
+				vendor::greater_than_t<Base, vendor::wrap_operand_t<T>> operator>(T t) const
+				{
+					using rhs = vendor::wrap_operand_t<T>;
+					static_assert(_is_valid_comparison_operand<rhs>::value, "invalid rhs operand in comparison");
+
+					return { *static_cast<const Base*>(this), rhs{t} };
+				}
+
+			template<typename T>
+				vendor::greater_equal_t<Base, vendor::wrap_operand_t<T>> operator>=(T t) const
+				{
+					using rhs = vendor::wrap_operand_t<T>;
+					static_assert(_is_valid_comparison_operand<rhs>::value, "invalid rhs operand in comparison");
+
+					return { *static_cast<const Base*>(this), rhs{t} };
+				}
+
+			vendor::is_null_t<true, Base> is_null() const
+			{
+				return { *static_cast<const Base*>(this) };
+			}
+
+			vendor::is_null_t<false, Base> is_not_null() const
+			{
+				return { *static_cast<const Base*>(this) };
+			}
+
+			sort_order_t<Base, sort_type::asc> asc()
+			{ 
+				return { *static_cast<const Base*>(this) };
+			}
+
+			sort_order_t<Base, sort_type::desc> desc()
+			{ 
+				return { *static_cast<const Base*>(this) };
+			}
+
+			// Hint: use value_list wrapper for containers...
+			template<typename... T>
+				vendor::in_t<true, Base, vendor::wrap_operand_t<T>...> in(T... t) const
+				{
+					static_assert(detail::all_t<_is_valid_comparison_operand, vendor::wrap_operand_t<T>...>::value, "at least one operand of in() is not valid");
+					return { *static_cast<const Base*>(this), vendor::wrap_operand_t<T>{t}... };
+				}
+
+			template<typename... T>
+				vendor::in_t<false, Base, vendor::wrap_operand_t<T>...> not_in(T... t) const
+				{
+					static_assert(detail::all_t<_is_valid_comparison_operand, vendor::wrap_operand_t<T>...>::value, "at least one operand of in() is not valid");
+					return { *static_cast<const Base*>(this), vendor::wrap_operand_t<T>{t}... };
+				}
+
+			template<typename alias_provider>
+				expression_alias_t<Base, alias_provider> as(const alias_provider&)
+				{
+					return { *static_cast<const Base*>(this) };
+				}
+		};
+
+}
+#endif
diff --git a/include/sqlpp11/basic_operators.h b/include/sqlpp11/basic_operators.h
deleted file mode 100644
index b832ffbf..00000000
--- a/include/sqlpp11/basic_operators.h
+++ /dev/null
@@ -1,137 +0,0 @@
-/*
- * Copyright (c) 2013, 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_DETAIL_BASIC_OPERATORS_H
-#define SQLPP_DETAIL_BASIC_OPERATORS_H
-
-#include <sqlpp11/alias.h>
-#include <sqlpp11/sort_order.h>
-#include <sqlpp11/vendor/expression_fwd.h>
-#include <sqlpp11/vendor/in_fwd.h>
-#include <sqlpp11/vendor/is_null_fwd.h>
-
-namespace sqlpp
-{
-
-	namespace detail
-	{
-		struct boolean;
-	}
-
-	// basic operators
-	template<typename Base, template<typename> class Constraint>
-		struct basic_operators
-		{
-			template<typename T>
-				vendor::equal_to_t<Base, typename Constraint<T>::type> operator==(T t) const
-				{
-					static_assert(not is_multi_expression_t<Base>::value, "multi-expression cannot be used as left hand side operand");
-					return { *static_cast<const Base*>(this), {t}	};
-				}
-
-			template<typename T>
-				vendor::not_equal_to_t<Base, typename Constraint<T>::type> operator!=(T t) const
-				{
-					static_assert(not is_multi_expression_t<Base>::value, "multi-expression cannot be used as left hand side operand");
-					return { *static_cast<const Base*>(this), {t} };
-				}
-			template<typename T>
-				vendor::less_than_t<Base, typename Constraint<T>::type> operator<(T t) const
-				{
-					static_assert(not is_multi_expression_t<Base>::value, "multi-expression cannot be used as left hand side operand");
-					return { *static_cast<const Base*>(this), {t} };
-				}
-
-			template<typename T>
-				vendor::less_equal_t<Base, typename Constraint<T>::type> operator<=(T t) const
-				{
-					static_assert(not is_multi_expression_t<Base>::value, "multi-expression cannot be used as left hand side operand");
-					return { *static_cast<const Base*>(this), {t} };
-				}
-
-			template<typename T>
-				vendor::greater_than_t<Base, typename Constraint<T>::type> operator>(T t) const
-				{
-					static_assert(not is_multi_expression_t<Base>::value, "multi-expression cannot be used as left hand side operand");
-					return { *static_cast<const Base*>(this), {t} };
-				}
-
-			template<typename T>
-				vendor::greater_equal_t<Base, typename Constraint<T>::type> operator>=(T t) const
-				{
-					static_assert(not is_multi_expression_t<Base>::value, "multi-expression cannot be used as left hand side operand");
-					return { *static_cast<const Base*>(this), {t} };
-				}
-
-			vendor::is_null_t<true, Base> is_null() const
-			{
-				static_assert(not is_multi_expression_t<Base>::value, "multi-expression cannot be used with is_null()");
-				return { *static_cast<const Base*>(this) };
-			}
-
-			vendor::is_null_t<false, Base> is_not_null() const
-			{
-				static_assert(not is_multi_expression_t<Base>::value, "multi-expression cannot be used with is_not_null()");
-				return { *static_cast<const Base*>(this) };
-			}
-
-			sort_order_t<Base, sort_type::asc> asc()
-			{ 
-				static_assert(not is_multi_expression_t<Base>::value, "multi-expression cannot be used for sorting");
-				return { *static_cast<const Base*>(this) };
-			}
-
-			sort_order_t<Base, sort_type::desc> desc()
-			{ 
-				static_assert(not is_multi_expression_t<Base>::value, "multi-expression cannot be used for sorting");
-				return { *static_cast<const Base*>(this) };
-			}
-
-			// Hint: use value_list wrapper for containers...
-			template<typename... T>
-				vendor::in_t<true, Base, typename Constraint<T>::type...> in(T... t) const
-				{
-					static_assert(not is_multi_expression_t<Base>::value, "multi-expression cannot be used with in()");
-					return { *static_cast<const Base*>(this), {t}... };
-				}
-
-			template<typename... T>
-				vendor::in_t<false, Base, typename Constraint<T>::type...> not_in(T... t) const
-				{
-					static_assert(not is_multi_expression_t<Base>::value, "multi-expression cannot with be used with not_in()");
-					return { *static_cast<const Base*>(this), {t}... };
-				}
-
-			template<typename alias_provider>
-				expression_alias_t<Base, alias_provider> as(const alias_provider&)
-				{
-					static_assert(not is_multi_expression_t<Base>::value, "multi-expression cannot have a name");
-					return { *static_cast<const Base*>(this) };
-				}
-		};
-
-}
-#endif
diff --git a/include/sqlpp11/boolean.h b/include/sqlpp11/boolean.h
index 117d75b9..27f2ed5c 100644
--- a/include/sqlpp11/boolean.h
+++ b/include/sqlpp11/boolean.h
@@ -30,7 +30,7 @@
 #include <cstdlib>
 #include <cassert>
 #include <ostream>
-#include <sqlpp11/basic_operators.h>
+#include <sqlpp11/basic_expression_operators.h>
 #include <sqlpp11/type_traits.h>
 #include <sqlpp11/exception.h>
 
@@ -175,33 +175,45 @@ namespace sqlpp
 			};
 
 			template<typename T>
-				using _operand_t = operand_t<T, is_boolean_t>;
-			template<typename T>
-				using _constraint = is_boolean_t<T>;
+				struct _is_valid_operand
+			{
+				static constexpr bool value = 
+					is_expression_t<T>::value // expressions are OK
+					and is_boolean_t<T>::value // the correct value type is required, of course
+					;
+			};
 
 			template<typename Base>
-				struct operators: public basic_operators<Base, _operand_t>
+				struct expression_operators: public basic_expression_operators<Base, is_boolean_t>
 			{
 				template<typename T>
-					vendor::logical_and_t<Base, typename _operand_t<T>::type> operator and(T t) const
+					vendor::logical_and_t<Base, vendor::wrap_operand_t<T>> operator and(T t) const
 					{
-						static_assert(not is_multi_expression_t<Base>::value, "multi-expression cannot be used as left hand side operand");
-						return { *static_cast<const Base*>(this), {t} };
+						using rhs = vendor::wrap_operand_t<T>;
+						static_assert(_is_valid_operand<rhs>::value, "invalid rhs operand");
+
+						return { *static_cast<const Base*>(this), rhs{t} };
 					}
 
 				template<typename T>
-					vendor::logical_or_t<Base, typename _operand_t<T>::type> operator or(T t) const
+					vendor::logical_or_t<Base, vendor::wrap_operand_t<T>> operator or(T t) const
 					{
-						static_assert(not is_multi_expression_t<Base>::value, "multi-expression cannot be used as left hand side operand");
-						return { *static_cast<const Base*>(this), {t} };
+						using rhs = vendor::wrap_operand_t<T>;
+						static_assert(_is_valid_operand<rhs>::value, "invalid rhs operand");
+
+						return { *static_cast<const Base*>(this), rhs{t} };
 					}
 
 				vendor::logical_not_t<Base> operator not() const
 				{
-					static_assert(not is_multi_expression_t<Base>::value, "multi-expression cannot be as operand for operator not");
 					return { *static_cast<const Base*>(this) };
 				}
 			};
+
+			template<typename Base>
+				struct column_operators
+			{
+			};
 		};
 
 		template<typename Db, bool TrivialIsNull>
diff --git a/include/sqlpp11/column.h b/include/sqlpp11/column.h
index 208ae1e0..5a3bd98a 100644
--- a/include/sqlpp11/column.h
+++ b/include/sqlpp11/column.h
@@ -42,7 +42,8 @@
 namespace sqlpp
 {
 	template<typename Table, typename ColumnSpec>
-	struct column_t: public ColumnSpec::_value_type::template operators<column_t<Table, ColumnSpec>>
+	struct column_t: public ColumnSpec::_value_type::template expression_operators<column_t<Table, ColumnSpec>>,
+	                 public ColumnSpec::_value_type::template column_operators<column_t<Table, ColumnSpec>>
 	{ 
 		using _is_column = std::true_type;
 		using _spec_t = ColumnSpec;
@@ -55,6 +56,8 @@ namespace sqlpp
 			using _is_named_expression = std::true_type;
 			using _is_alias = std::false_type;
 		};
+		template<typename T>
+			using _is_valid_operand = typename _value_type::template _is_valid_operand<T>;
 
 		using _name_t = typename ColumnSpec::_name_t;
 
@@ -77,19 +80,12 @@ namespace sqlpp
 			}
 
 		template<typename T>
-			auto operator =(T t) const
-			-> typename std::enable_if<_value_type::template _constraint<typename vendor::wrap_operand<T>::type>::value and not std::is_same<column_t, T>::value, 
-			     vendor::assignment_t<column_t, typename vendor::wrap_operand<T>::type>>::type
+			auto operator =(T t) const -> vendor::assignment_t<column_t, typename vendor::wrap_operand<T>::type>
 			{
-				return { *this, {t} };
-			}
+				using rhs = vendor::wrap_operand_t<T>;
+				static_assert(_is_valid_operand<rhs>::value, "invalid rhs operand assignment operand");
 
-		template<typename T>
-			auto operator =(T t) const
-			-> typename std::enable_if<not(_value_type::template _constraint<typename vendor::wrap_operand<T>::type>::value and not std::is_same<column_t, T>::value),
-			     vendor::assignment_t<column_t, typename vendor::wrap_operand<T>::type>>::type
-			{
-				static_assert(sqlpp::vendor::wrong_t<T>::value, "invalid assignment operand");
+				return { *this, {t} };
 			}
 
 		auto operator =(sqlpp::null_t) const
diff --git a/include/sqlpp11/count.h b/include/sqlpp11/count.h
index f6253ebf..3a8de5a9 100644
--- a/include/sqlpp11/count.h
+++ b/include/sqlpp11/count.h
@@ -35,10 +35,10 @@ namespace sqlpp
 	namespace vendor
 	{
 		template<typename Flag, typename Expr>
-		struct count_t: public sqlpp::detail::integral::template operators<count_t<Flag, Expr>>
+		struct count_t: public sqlpp::detail::integral::template expression_operators<count_t<Flag, Expr>>
 		{
 			static_assert(is_noop<Flag>::value or std::is_same<sqlpp::distinct_t, Flag>::value, "count() used with flag other than 'distinct'");
-			static_assert(is_value_t<Expr>::value, "count() requires a sql value as argument");
+			static_assert(is_expression_t<Expr>::value, "count() requires a sql expression as argument");
 
 			struct _value_type: public sqlpp::detail::integral
 			{
@@ -94,14 +94,16 @@ namespace sqlpp
 	}
 
 	template<typename T>
-		auto count(T t) -> typename vendor::count_t<vendor::noop, typename operand_t<T, is_value_t>::type>
+		auto count(T t) -> typename vendor::count_t<vendor::noop, vendor::wrap_operand_t<T>>
 		{
+			static_assert(is_expression_t<vendor::wrap_operand_t<T>>::value, "count() requires an expression as argument");
 			return { t };
 		}
 
 	template<typename T>
-		auto count(const sqlpp::distinct_t&, T t) -> typename vendor::count_t<sqlpp::distinct_t, typename operand_t<T, is_value_t>::type>
+		auto count(const sqlpp::distinct_t&, T t) -> typename vendor::count_t<sqlpp::distinct_t, vendor::wrap_operand_t<T>>
 		{
+			static_assert(is_expression_t<vendor::wrap_operand_t<T>>::value, "count() requires an expression as argument");
 			return { t };
 		}
 
diff --git a/include/sqlpp11/exists.h b/include/sqlpp11/exists.h
index 56fb925b..a63af7f0 100644
--- a/include/sqlpp11/exists.h
+++ b/include/sqlpp11/exists.h
@@ -34,7 +34,7 @@ namespace sqlpp
 	namespace vendor
 	{
 		template<typename Select>
-		struct exists_t: public boolean::template operators<exists_t<Select>>
+		struct exists_t: public boolean::template expression_operators<exists_t<Select>>
 		{
 			static_assert(is_select_t<Select>::value, "exists() requires a select expression as argument");
 
@@ -88,8 +88,9 @@ namespace sqlpp
 
 
 	template<typename T>
-		auto exists(T t) -> typename vendor::exists_t<typename operand_t<T, is_select_t>::type>
+		auto exists(T t) -> typename vendor::exists_t<vendor::wrap_operand_t<T>>
 		{
+			static_assert(is_select_t<vendor::wrap_operand_t<T>>::value, "exists() requires a select expression as argument");
 			return { t };
 		}
 
diff --git a/include/sqlpp11/floating_point.h b/include/sqlpp11/floating_point.h
index 4f8a47e8..8ea533c7 100644
--- a/include/sqlpp11/floating_point.h
+++ b/include/sqlpp11/floating_point.h
@@ -29,7 +29,7 @@
 
 #include <cstdlib>
 #include <cassert>
-#include <sqlpp11/basic_operators.h>
+#include <sqlpp11/basic_expression_operators.h>
 #include <sqlpp11/type_traits.h>
 #include <sqlpp11/exception.h>
 
@@ -177,78 +177,100 @@ namespace sqlpp
 			};
 
 			template<typename T>
-				using _operand_t = operand_t<T, is_numeric_t>;
-			template<typename T>
-				using _constraint = is_numeric_t<T>;
+				struct _is_valid_operand
+			{
+				static constexpr bool value = 
+					is_expression_t<T>::value // expressions are OK
+					and is_numeric_t<T>::value // the correct value type is required, of course
+					;
+			};
 
 			template<typename Base>
-				struct operators: public basic_operators<Base, _operand_t>
+				struct expression_operators: public basic_expression_operators<Base, is_numeric_t>
 			{
 				template<typename T>
-					vendor::plus_t<Base, floating_point, typename _operand_t<T>::type> operator +(T t) const
+					vendor::plus_t<Base, floating_point, vendor::wrap_operand_t<T>> operator +(T t) const
 					{
-						static_assert(not is_multi_expression_t<Base>::value, "multi-expression cannot be used as left hand side operand");
-						return { *static_cast<const Base*>(this), {t} };
+						using rhs = vendor::wrap_operand_t<T>;
+						static_assert(_is_valid_operand<rhs>::value, "invalid rhs operand");
+
+						return { *static_cast<const Base*>(this), rhs{t} };
 					}
 
 				template<typename T>
-					vendor::minus_t<Base, floating_point, typename _operand_t<T>::type> operator -(T t) const
+					vendor::minus_t<Base, floating_point, vendor::wrap_operand_t<T>> operator -(T t) const
 					{
-						static_assert(not is_multi_expression_t<Base>::value, "multi-expression cannot be used as left hand side operand");
-						return { *static_cast<const Base*>(this), {t} };
+						using rhs = vendor::wrap_operand_t<T>;
+						static_assert(_is_valid_operand<rhs>::value, "invalid rhs operand");
+
+						return { *static_cast<const Base*>(this), rhs{t} };
 					}
 
 				template<typename T>
-					vendor::multiplies_t<Base, floating_point, typename _operand_t<T>::type> operator *(T t) const
+					vendor::multiplies_t<Base, floating_point, vendor::wrap_operand_t<T>> operator *(T t) const
 					{
-						static_assert(not is_multi_expression_t<Base>::value, "multi-expression cannot be used as left hand side operand");
-						return { *static_cast<const Base*>(this), {t} };
+						using rhs = vendor::wrap_operand_t<T>;
+
+						return { *static_cast<const Base*>(this), rhs{t} };
 					}
 
 				template<typename T>
-					vendor::divides_t<Base, typename _operand_t<T>::type> operator /(T t) const
+					vendor::divides_t<Base, vendor::wrap_operand_t<T>> operator /(T t) const
 					{
-						static_assert(not is_multi_expression_t<Base>::value, "multi-expression cannot be used as left hand side operand");
-						return { *static_cast<const Base*>(this), {t} };
+						using rhs = vendor::wrap_operand_t<T>;
+
+						return { *static_cast<const Base*>(this), rhs{t} };
 					}
 
 				vendor::unary_plus_t<floating_point, Base> operator +() const
 				{
-					static_assert(not is_multi_expression_t<Base>::value, "multi-expression cannot be used as unary operand");
 					return { *static_cast<const Base*>(this) };
 				}
 
 				vendor::unary_minus_t<floating_point, Base> operator -() const
 				{
-					static_assert(not is_multi_expression_t<Base>::value, "multi-expression cannot be used as unary operand");
 					return { *static_cast<const Base*>(this) };
 				}
+			};
 
+			template<typename Base>
+				struct column_operators
+			{
 				template<typename T>
-					auto operator +=(T t) const -> decltype(std::declval<Base>() = std::declval<Base>() + t)
+					auto operator +=(T t) const -> vendor::assignment_t<Base, vendor::plus_t<Base, floating_point, vendor::wrap_operand_t<T>>>
 					{
-						return *static_cast<const Base*>(this) = operator +(t);
+						using rhs = vendor::wrap_operand_t<T>;
+						static_assert(_is_valid_operand<rhs>::value, "invalid rhs assignment operand");
+
+						return { *static_cast<const Base*>(this), { *static_cast<const Base*>(this), rhs{t} } };
 					}
 
 				template<typename T>
-					auto operator -=(T t) const -> decltype(std::declval<Base>() = std::declval<Base>() - t)
+					auto operator -=(T t) const -> vendor::assignment_t<Base, vendor::minus_t<Base, floating_point, vendor::wrap_operand_t<T>>>
 					{
-						return *static_cast<const Base*>(this) = operator -(t);
+						using rhs = vendor::wrap_operand_t<T>;
+						static_assert(_is_valid_operand<rhs>::value, "invalid rhs assignment operand");
+
+						return { *static_cast<const Base*>(this), { *static_cast<const Base*>(this), rhs{t} } };
 					}
 
 				template<typename T>
-					auto operator /=(T t) const -> decltype(std::declval<Base>() = std::declval<Base>() / t)
+					auto operator /=(T t) const -> vendor::assignment_t<Base, vendor::divides_t<Base, vendor::wrap_operand_t<T>>>
 					{
-						return *static_cast<const Base*>(this) = operator /(t);
+						using rhs = vendor::wrap_operand_t<T>;
+						static_assert(_is_valid_operand<rhs>::value, "invalid rhs assignment operand");
+
+						return { *static_cast<const Base*>(this), { *static_cast<const Base*>(this), rhs{t} } };
 					}
 
 				template<typename T>
-					auto operator *=(T t) const -> decltype(std::declval<Base>() = std::declval<Base>() * t)
+					auto operator *=(T t) const -> vendor::assignment_t<Base, vendor::multiplies_t<Base, floating_point, vendor::wrap_operand_t<T>>>
 					{
-						return *static_cast<const Base*>(this) = operator *(t);
+						using rhs = vendor::wrap_operand_t<T>;
+						static_assert(_is_valid_operand<rhs>::value, "invalid rhs assignment operand");
+
+						return { *static_cast<const Base*>(this), { *static_cast<const Base*>(this), rhs{t} } };
 					}
-
-
 			};
 		};
 
diff --git a/include/sqlpp11/functions.h b/include/sqlpp11/functions.h
index cdd556fa..caf8097d 100644
--- a/include/sqlpp11/functions.h
+++ b/include/sqlpp11/functions.h
@@ -32,6 +32,7 @@
 #include <sqlpp11/column_types.h>
 #include <sqlpp11/vendor/in.h>
 #include <sqlpp11/vendor/is_null.h>
+#include <sqlpp11/vendor/value_type.h>
 #include <sqlpp11/exists.h>
 #include <sqlpp11/any.h>
 #include <sqlpp11/some.h>
@@ -45,7 +46,7 @@
 namespace sqlpp
 {
 	template<typename T>
-		auto value(T t) -> typename operand_t<T, is_value_t>::type
+		auto value(T t) -> vendor::wrap_operand_t<T>
 		{
 			using _table_set = ::sqlpp::detail::type_set<>;
 			static_assert(not is_value_t<T>::value, "value() is to be called with non-sql-type like int, or string");
@@ -53,7 +54,7 @@ namespace sqlpp
 		}
 
 	template<typename ValueType> // Csaba Csoma suggests: unsafe_sql instead of verbatim
-	struct verbatim_t: public ValueType::template operators<verbatim_t<ValueType>>
+	struct verbatim_t: public ValueType::template expression_operators<verbatim_t<ValueType>>
 	{
 		using _value_type = ValueType;
 		using _table_set = ::sqlpp::detail::type_set<>;
@@ -103,7 +104,7 @@ namespace sqlpp
 		{
 			using _container_t = Container;
 			using _table_set = ::sqlpp::detail::type_set<>;// FIXME: Could it be something else?
-			using _value_type = typename operand_t<typename _container_t::value_type, is_value_t>::type::_value_type;
+			using _value_type = vendor::value_type_t<typename _container_t::value_type>;
 
 			value_list_t(_container_t container):
 				_container(container)
diff --git a/include/sqlpp11/integral.h b/include/sqlpp11/integral.h
index a143e5de..a90bc76d 100644
--- a/include/sqlpp11/integral.h
+++ b/include/sqlpp11/integral.h
@@ -29,7 +29,7 @@
 
 #include <cstdlib>
 #include <cassert>
-#include <sqlpp11/basic_operators.h>
+#include <sqlpp11/basic_expression_operators.h>
 #include <sqlpp11/type_traits.h>
 #include <sqlpp11/exception.h>
 #include <sqlpp11/vendor/value_type.h>
@@ -177,85 +177,111 @@ namespace sqlpp
 			};
 
 			template<typename T>
-				using _operand_t = operand_t<T, is_numeric_t>;
-			template<typename T>
-				using _constraint = is_numeric_t<T>;
+				struct _is_valid_operand
+			{
+				static constexpr bool value = 
+					is_expression_t<T>::value // expressions are OK
+					and is_numeric_t<T>::value // the correct value type is required, of course
+					;
+			};
 
 			template<typename Base>
-				struct operators: public basic_operators<Base, _operand_t>
+				struct expression_operators: public basic_expression_operators<Base, is_numeric_t>
 			{
 				template<typename T>
-					vendor::plus_t<Base, vendor::value_type_t<T>, typename _operand_t<T>::type> operator +(T t) const
+					vendor::plus_t<Base, vendor::value_type_t<T>, vendor::wrap_operand_t<T>> operator +(T t) const
 					{
-						static_assert(not is_multi_expression_t<Base>::value, "multi-expression cannot be used as left hand side operand");
+						using rhs = vendor::wrap_operand_t<T>;
+						static_assert(_is_valid_operand<rhs>::value, "invalid rhs operand");
+
 						return { *static_cast<const Base*>(this), {t} };
 					}
 
 				template<typename T>
-					vendor::minus_t<Base, vendor::value_type_t<T>, typename _operand_t<T>::type> operator -(T t) const
+					vendor::minus_t<Base, vendor::value_type_t<T>, vendor::wrap_operand_t<T>> operator -(T t) const
 					{
-						static_assert(not is_multi_expression_t<Base>::value, "multi-expression cannot be used as left hand side operand");
+						using rhs = vendor::wrap_operand_t<T>;
+						static_assert(_is_valid_operand<rhs>::value, "invalid rhs operand");
+
 						return { *static_cast<const Base*>(this), {t} };
 					}
 
 				template<typename T>
-					vendor::multiplies_t<Base, vendor::value_type_t<T>, typename _operand_t<T>::type> operator *(T t) const
+					vendor::multiplies_t<Base, vendor::value_type_t<T>, vendor::wrap_operand_t<T>> operator *(T t) const
 					{
-						static_assert(not is_multi_expression_t<Base>::value, "multi-expression cannot be used as left hand side operand");
+						using rhs = vendor::wrap_operand_t<T>;
+						static_assert(_is_valid_operand<rhs>::value, "invalid rhs operand");
+
 						return { *static_cast<const Base*>(this), {t} };
 					}
 
 				template<typename T>
-					vendor::divides_t<Base, typename _operand_t<T>::type> operator /(T t) const
+					vendor::divides_t<Base, vendor::wrap_operand_t<T>> operator /(T t) const
 					{
-						static_assert(not is_multi_expression_t<Base>::value, "multi-expression cannot be used as left hand side operand");
+						using rhs = vendor::wrap_operand_t<T>;
+						static_assert(_is_valid_operand<rhs>::value, "invalid rhs operand");
+
 						return { *static_cast<const Base*>(this), {t} };
 					}
 
 				template<typename T>
-					vendor::modulus_t<Base, typename _operand_t<T>::type> operator %(T t) const
+					vendor::modulus_t<Base, vendor::wrap_operand_t<T>> operator %(T t) const
 					{
-						static_assert(not is_multi_expression_t<Base>::value, "multi-expression cannot be used as left hand side operand");
+						using rhs = vendor::wrap_operand_t<T>;
+						static_assert(_is_valid_operand<rhs>::value, "invalid rhs operand");
+
 						return { *static_cast<const Base*>(this), {t} };
 					}
 
 				vendor::unary_plus_t<integral, Base> operator +() const
 				{
-					static_assert(not is_multi_expression_t<Base>::value, "multi-expression cannot be used as unary operand");
 					return { *static_cast<const Base*>(this) };
 				}
 
 				vendor::unary_minus_t<integral, Base> operator -() const
 				{
-					static_assert(not is_multi_expression_t<Base>::value, "multi-expression cannot be used as unary operand");
 					return { *static_cast<const Base*>(this) };
 				}
+			};
 
+			template<typename Base>
+				struct column_operators
+			{
 				template<typename T>
-					auto operator +=(T t) const -> decltype(std::declval<Base>() = std::declval<Base>() + t)
+					auto operator +=(T t) const -> vendor::assignment_t<Base, vendor::plus_t<Base, vendor::value_type_t<T>, vendor::wrap_operand_t<T>>>
 					{
-						return *static_cast<const Base*>(this) = operator +(t);
+						using rhs = vendor::wrap_operand_t<T>;
+						static_assert(_is_valid_operand<rhs>::value, "invalid rhs assignment operand");
+
+						return { *static_cast<const Base*>(this), { *static_cast<const Base*>(this), rhs{t} } };
 					}
 
 				template<typename T>
-					auto operator -=(T t) const -> decltype(std::declval<Base>() = std::declval<Base>() - t)
+					auto operator -=(T t) const -> vendor::assignment_t<Base, vendor::minus_t<Base, vendor::value_type_t<T>, vendor::wrap_operand_t<T>>>
 					{
-						return *static_cast<const Base*>(this) = operator -(t);
+						using rhs = vendor::wrap_operand_t<T>;
+						static_assert(_is_valid_operand<rhs>::value, "invalid rhs assignment operand");
+
+						return { *static_cast<const Base*>(this), { *static_cast<const Base*>(this), rhs{t} } };
 					}
 
 				template<typename T>
-					auto operator /=(T t) const -> decltype(std::declval<Base>() = std::declval<Base>() / t)
+					auto operator /=(T t) const -> vendor::assignment_t<Base, vendor::divides_t<Base, vendor::wrap_operand_t<T>>>
 					{
-						return *static_cast<const Base*>(this) = operator /(t);
+						using rhs = vendor::wrap_operand_t<T>;
+						static_assert(_is_valid_operand<rhs>::value, "invalid rhs assignment operand");
+
+						return { *static_cast<const Base*>(this), { *static_cast<const Base*>(this), rhs{t} } };
 					}
 
 				template<typename T>
-					auto operator *=(T t) const -> decltype(std::declval<Base>() = std::declval<Base>() * t)
+					auto operator *=(T t) const -> vendor::assignment_t<Base, vendor::multiplies_t<Base, vendor::value_type_t<T>, vendor::wrap_operand_t<T>>>
 					{
-						return *static_cast<const Base*>(this) = operator *(t);
+						using rhs = vendor::wrap_operand_t<T>;
+						static_assert(_is_valid_operand<rhs>::value, "invalid rhs assignment operand");
+
+						return { *static_cast<const Base*>(this), { *static_cast<const Base*>(this), rhs{t} } };
 					}
-
-
 			};
 		};
 
diff --git a/include/sqlpp11/max.h b/include/sqlpp11/max.h
index 241ab544..fa4bbdc4 100644
--- a/include/sqlpp11/max.h
+++ b/include/sqlpp11/max.h
@@ -34,7 +34,7 @@ namespace sqlpp
 	namespace vendor
 	{
 		template<typename Expr>
-		struct max_t: public boolean::template operators<max_t<Expr>>
+		struct max_t: public Expr::_value_type::template expression_operators<max_t<Expr>>
 		{
 			static_assert(is_value_t<Expr>::value, "max() requires a value expression as argument");
 
@@ -87,8 +87,9 @@ namespace sqlpp
 	}
 
 	template<typename T>
-		auto max(T t) -> typename vendor::max_t<typename operand_t<T, is_value_t>::type>
+		auto max(T t) -> typename vendor::max_t<vendor::wrap_operand_t<T>>
 		{
+			static_assert(is_value_t<vendor::wrap_operand_t<T>>::value, "max() requires a value expression as argument");
 			return { t };
 		}
 
diff --git a/include/sqlpp11/min.h b/include/sqlpp11/min.h
index 4fd7d5c8..7062f778 100644
--- a/include/sqlpp11/min.h
+++ b/include/sqlpp11/min.h
@@ -34,7 +34,7 @@ namespace sqlpp
 	namespace vendor
 	{
 		template<typename Expr>
-		struct min_t: public boolean::template operators<min_t<Expr>>
+		struct min_t: public Expr::_value_type::template expression_operators<min_t<Expr>>
 		{
 			static_assert(is_value_t<Expr>::value, "min() requires a value expression as argument");
 
@@ -87,8 +87,9 @@ namespace sqlpp
 	}
 
 	template<typename T>
-		auto min(T t) -> typename vendor::min_t<typename operand_t<T, is_value_t>::type>
+		auto min(T t) -> typename vendor::min_t<vendor::wrap_operand_t<T>>
 		{
+			static_assert(is_value_t<vendor::wrap_operand_t<T>>::value, "min() requires a value expression as argument");
 			return { t };
 		}
 
diff --git a/include/sqlpp11/no_value.h b/include/sqlpp11/no_value.h
index 2f330147..eb95d7b9 100644
--- a/include/sqlpp11/no_value.h
+++ b/include/sqlpp11/no_value.h
@@ -36,8 +36,19 @@ namespace sqlpp
 		template<typename T>
 			using _constraint = std::false_type;
 
+		template<typename T>
+			struct _is_valid_operand
+			{
+				static constexpr bool value = false;
+			};
+
 		template<typename Base>
-			struct operators
+			struct expression_operators
+			{
+			};
+
+		template<typename Base>
+			struct column_operators
 			{
 			};
 	};
diff --git a/include/sqlpp11/parameter.h b/include/sqlpp11/parameter.h
index 260dc88a..8029ad02 100644
--- a/include/sqlpp11/parameter.h
+++ b/include/sqlpp11/parameter.h
@@ -33,11 +33,15 @@
 namespace sqlpp
 {
 	template<typename ValueType, typename NameType>
-	struct parameter_t: public ValueType::template operators<parameter_t<ValueType, NameType>>
+	struct parameter_t: public ValueType::template expression_operators<parameter_t<ValueType, NameType>>
 	{
-		using _value_type = ValueType;
+#warning need to check that Value Type is an SQL value type!
+		struct _value_type: public ValueType
+		{
+			using _is_expression = std::true_type;
+			using _is_alias = std::false_type;
+		};
 		using _is_parameter = std::true_type;
-		using _is_expression_t = std::true_type;
 		using _instance_t = typename NameType::_name_t::template _member_t<typename ValueType::_parameter_t>;
 		using _table_set = sqlpp::detail::type_set<>;
 
diff --git a/include/sqlpp11/select.h b/include/sqlpp11/select.h
index 44df0227..009d5202 100644
--- a/include/sqlpp11/select.h
+++ b/include/sqlpp11/select.h
@@ -80,7 +80,7 @@ namespace sqlpp
 			typename Limit = vendor::no_limit_t, 
 			typename Offset = vendor::no_offset_t
 				>
-		struct select_t: public detail::select_helper_t<ColumnList, From>::_value_type::template operators<select_t<Database, FlagList, ColumnList, From, Where, GroupBy, Having, OrderBy, Limit, Offset>>
+		struct select_t: public detail::select_helper_t<ColumnList, From>::_value_type::template expression_operators<select_t<Database, FlagList, ColumnList, From, Where, GroupBy, Having, OrderBy, Limit, Offset>>
 		{
 			using _database_t = Database;
 			using _is_dynamic = typename std::conditional<std::is_same<Database, void>::value, std::false_type, std::true_type>::type;
diff --git a/include/sqlpp11/some.h b/include/sqlpp11/some.h
index 54a1fe5b..250d2bac 100644
--- a/include/sqlpp11/some.h
+++ b/include/sqlpp11/some.h
@@ -35,14 +35,15 @@ namespace sqlpp
 	namespace vendor
 	{
 		template<typename Select>
-		struct some_t: public boolean::template operators<some_t<Select>>
+		struct some_t
 		{
 			static_assert(is_select_t<Select>::value, "some() requires a single column select expression as argument");
 			static_assert(is_value_t<Select>::value, "some() requires a single column select expression as argument");
 
 			struct _value_type: public Select::_value_type::_base_value_type
 			{
-				using _is_multi_expression = std::true_type; // must not be named
+				using _is_expression = std::false_type;
+				using _is_multi_expression = std::true_type; // must not be named or used with +,-,*,/, etc
 			};
 
 			struct _name_t
@@ -90,8 +91,9 @@ namespace sqlpp
 	}
 
 	template<typename T>
-		auto some(T t) -> typename vendor::some_t<typename operand_t<T, is_select_t>::type>
+		auto some(T t) -> typename vendor::some_t<vendor::wrap_operand_t<T>>
 		{
+			static_assert(is_select_t<vendor::wrap_operand_t<T>>::value, "some() requires a select expression as argument");
 			return { t };
 		}
 
diff --git a/include/sqlpp11/sum.h b/include/sqlpp11/sum.h
index 0a2ca27e..168cbfb5 100644
--- a/include/sqlpp11/sum.h
+++ b/include/sqlpp11/sum.h
@@ -34,7 +34,7 @@ namespace sqlpp
 	namespace vendor
 	{
 		template<typename Flag, typename Expr>
-		struct sum_t: public boolean::template operators<sum_t<Flag, Expr>>
+		struct sum_t: public Expr::_value_type::template expression_operators<sum_t<Flag, Expr>>
 		{
 			static_assert(is_noop<Flag>::value or std::is_same<sqlpp::distinct_t, Flag>::value, "sum() used with flag other than 'distinct'");
 			static_assert(is_numeric_t<Expr>::value, "sum() requires a numeric expression as argument");
@@ -93,14 +93,16 @@ namespace sqlpp
 	}
 
 	template<typename T>
-		auto sum(T t) -> typename vendor::sum_t<vendor::noop, typename operand_t<T, is_value_t>::type>
+		auto sum(T t) -> typename vendor::sum_t<vendor::noop, vendor::wrap_operand_t<T>>
 		{
+			static_assert(is_numeric_t<vendor::wrap_operand_t<T>>::value, "sum() requires a numeric expression as argument");
 			return { t };
 		}
 
 	template<typename T>
-		auto sum(const sqlpp::distinct_t&, T t) -> typename vendor::sum_t<sqlpp::distinct_t, typename operand_t<T, is_value_t>::type>
+		auto sum(const sqlpp::distinct_t&, T t) -> typename vendor::sum_t<sqlpp::distinct_t, vendor::wrap_operand_t<T>>
 		{
+			static_assert(is_numeric_t<vendor::wrap_operand_t<T>>::value, "sum() requires a numeric expression as argument");
 			return { t };
 		}
 
diff --git a/include/sqlpp11/text.h b/include/sqlpp11/text.h
index b16f4ea8..36d41b60 100644
--- a/include/sqlpp11/text.h
+++ b/include/sqlpp11/text.h
@@ -28,7 +28,7 @@
 #define SQLPP_TEXT_H
 
 #include <cassert>
-#include <sqlpp11/basic_operators.h>
+#include <sqlpp11/basic_expression_operators.h>
 #include <sqlpp11/type_traits.h>
 #include <sqlpp11/exception.h>
 #include <sqlpp11/vendor/concat.h>
@@ -180,33 +180,47 @@ namespace sqlpp
 			};
 
 			template<typename T>
-				using _operand_t = operand_t<T, is_text_t>;
-			template<typename T>
-				using _constraint = is_text_t<T>;
+				struct _is_valid_operand
+			{
+				static constexpr bool value = 
+					is_expression_t<T>::value // expressions are OK
+					and is_text_t<T>::value // the correct value type is required, of course
+					;
+			};
 
 			template<typename Base>
-				struct operators: public basic_operators<Base, _operand_t>
+				struct expression_operators: public basic_expression_operators<Base, is_text_t>
 			{
 				template<typename T>
-					vendor::concat_t<Base, typename _operand_t<T>::type> operator+(T t) const
+					vendor::concat_t<Base, vendor::wrap_operand_t<T>> operator+(T t) const
 					{
-						static_assert(not is_multi_expression_t<Base>::value, "multi-expression cannot be used as left hand side operand");
+						using rhs = vendor::wrap_operand_t<T>;
+						static_assert(_is_valid_operand<rhs>::value, "invalid rhs operand");
+
 						return { *static_cast<const Base*>(this), {t} };
 					}
 
 				template<typename T>
-					auto operator +=(T t) const -> decltype(std::declval<Base>() = std::declval<Base>() + t)
+					vendor::like_t<Base, vendor::wrap_operand_t<T>> like(T t) const
 					{
-						return *static_cast<const Base*>(this) = operator +(t);
-					}
+						using rhs = vendor::wrap_operand_t<T>;
+						static_assert(_is_valid_operand<rhs>::value, "invalid argument for like()");
 
-				template<typename T>
-					vendor::like_t<Base, typename _operand_t<T>::type> like(T t) const
-					{
-						static_assert(not is_multi_expression_t<Base>::value, "multi-expression cannot be used as left hand side operand");
 						return { *static_cast<const Base*>(this), {t} };
 					}
+			};
 
+			template<typename Base>
+				struct column_operators
+			{
+				template<typename T>
+					auto operator +=(T t) const -> vendor::assignment_t<Base, vendor::concat_t<Base, vendor::wrap_operand_t<T>>>
+					{
+						using rhs = vendor::wrap_operand_t<T>;
+						static_assert(_is_valid_operand<rhs>::value, "invalid rhs assignment operand");
+
+						return { *static_cast<const Base*>(this), { *static_cast<const Base*>(this), rhs{t} } };
+					}
 			};
 		};
 
diff --git a/include/sqlpp11/type_traits.h b/include/sqlpp11/type_traits.h
index 8ee87cfa..c0a380e7 100644
--- a/include/sqlpp11/type_traits.h
+++ b/include/sqlpp11/type_traits.h
@@ -129,16 +129,6 @@ namespace sqlpp
 
 	template<typename T, template<typename> class IsTag>
 		using copy_type_trait = typename std::conditional<IsTag<T>::value, std::true_type, std::false_type>::type;
-
-	template<typename T, template<typename> class IsCorrectType>
-		struct operand_t
-		{
-			using type = typename vendor::wrap_operand<T>::type;
-			static_assert(not is_alias_t<type>::value, "expression operand must not be an alias");
-			static_assert(is_expression_t<type>::value, "expression required");
-			static_assert(IsCorrectType<type>::value, "invalid operand type");
-		};
-	
 }
 
 #endif
diff --git a/include/sqlpp11/vendor/concat.h b/include/sqlpp11/vendor/concat.h
index fb8e97b4..fda3be94 100644
--- a/include/sqlpp11/vendor/concat.h
+++ b/include/sqlpp11/vendor/concat.h
@@ -36,7 +36,7 @@ namespace sqlpp
 	namespace vendor
 	{
 		template<typename First, typename... Args>
-			struct concat_t: public First::_value_type::template operators<concat_t<First, Args...>>
+			struct concat_t: public First::_value_type::template expression_operators<concat_t<First, Args...>>
 		{
 			static_assert(sizeof...(Args) > 0, "concat requires two arguments at least");
 			static_assert(sqlpp::detail::all_t<is_text_t, First, Args...>::value, "at least one non-text argument detected in concat()");
diff --git a/include/sqlpp11/vendor/expression.h b/include/sqlpp11/vendor/expression.h
index 455aebb7..b2be6368 100644
--- a/include/sqlpp11/vendor/expression.h
+++ b/include/sqlpp11/vendor/expression.h
@@ -40,7 +40,7 @@ namespace sqlpp
 	namespace vendor
 	{
 		template<typename Lhs, typename Rhs>
-			struct binary_expression_t<Lhs, tag::equal_to, Rhs>: public ::sqlpp::detail::boolean::template operators<equal_to_t<Lhs, Rhs>>
+			struct binary_expression_t<Lhs, tag::equal_to, Rhs>: public ::sqlpp::detail::boolean::template expression_operators<equal_to_t<Lhs, Rhs>>
 		{
 			using _value_type = ::sqlpp::detail::boolean;
 			using _parameter_tuple_t = std::tuple<Lhs, Rhs>;
@@ -85,7 +85,7 @@ namespace sqlpp
 			};
 
 		template<typename Lhs, typename Rhs>
-			struct binary_expression_t<Lhs, tag::not_equal_to, Rhs>: public ::sqlpp::detail::boolean::template operators<not_equal_to_t<Lhs, Rhs>>
+			struct binary_expression_t<Lhs, tag::not_equal_to, Rhs>: public ::sqlpp::detail::boolean::template expression_operators<not_equal_to_t<Lhs, Rhs>>
 		{
 			using _value_type = ::sqlpp::detail::boolean;
 			using _parameter_tuple_t = std::tuple<Lhs, Rhs>;
@@ -130,7 +130,7 @@ namespace sqlpp
 			};
 
 		template<typename Rhs>
-			struct unary_expression_t<tag::logical_not, Rhs>: public ::sqlpp::detail::boolean::template operators<logical_not_t<Rhs>>
+			struct unary_expression_t<tag::logical_not, Rhs>: public ::sqlpp::detail::boolean::template expression_operators<logical_not_t<Rhs>>
 		{
 			using _value_type = ::sqlpp::detail::boolean;
 			using _parameter_tuple_t = std::tuple<Rhs>;
@@ -165,7 +165,7 @@ namespace sqlpp
 			};
 
 		template<typename Lhs, typename O, typename Rhs>
-			struct binary_expression_t: public O::_value_type::template operators<binary_expression_t<Lhs, O, Rhs>>
+			struct binary_expression_t: public O::_value_type::template expression_operators<binary_expression_t<Lhs, O, Rhs>>
 		{
 			using _lhs_t = Lhs;
 			using _rhs_t = Rhs;
@@ -205,7 +205,7 @@ namespace sqlpp
 			};
 
 		template<typename O, typename Rhs>
-			struct unary_expression_t: public O::_value_type::template operators<unary_expression_t<O, Rhs>>
+			struct unary_expression_t: public O::_value_type::template expression_operators<unary_expression_t<O, Rhs>>
 		{
 			using _value_type = typename O::_value_type;
 			using _parameter_tuple_t = std::tuple<Rhs>;
diff --git a/include/sqlpp11/vendor/in.h b/include/sqlpp11/vendor/in.h
index 8dcfffa9..b3d37f5d 100644
--- a/include/sqlpp11/vendor/in.h
+++ b/include/sqlpp11/vendor/in.h
@@ -37,7 +37,7 @@ namespace sqlpp
 	namespace vendor
 	{
 		template<bool NotInverted, typename Operand, typename... Args>
-			struct in_t: public boolean::template operators<in_t<NotInverted, Operand, Args...>>
+			struct in_t: public boolean::template expression_operators<in_t<NotInverted, Operand, Args...>>
 		{
 			static constexpr bool _inverted = not NotInverted;
 			static_assert(sizeof...(Args) > 0, "in() requires at least one argument");
diff --git a/include/sqlpp11/vendor/is_null.h b/include/sqlpp11/vendor/is_null.h
index fb6c2e77..5abc096e 100644
--- a/include/sqlpp11/vendor/is_null.h
+++ b/include/sqlpp11/vendor/is_null.h
@@ -36,7 +36,7 @@ namespace sqlpp
 	namespace vendor
 	{
 		template<bool NotInverted, typename Operand>
-		struct is_null_t: public boolean::template operators<is_null_t<NotInverted, Operand>>
+		struct is_null_t: public boolean::template expression_operators<is_null_t<NotInverted, Operand>>
 		{
 			static constexpr bool _inverted = not NotInverted;
 			using _table_set = typename Operand::_table_set;
diff --git a/include/sqlpp11/vendor/like.h b/include/sqlpp11/vendor/like.h
index 24f1592e..a2f1755e 100644
--- a/include/sqlpp11/vendor/like.h
+++ b/include/sqlpp11/vendor/like.h
@@ -36,7 +36,7 @@ namespace sqlpp
 	namespace vendor
 	{
 		template<typename Operand, typename Pattern>
-			struct like_t: public boolean::template operators<like_t<Operand, Pattern>>
+			struct like_t: public boolean::template expression_operators<like_t<Operand, Pattern>>
 		{
 			static_assert(is_text_t<Operand>::value, "Operand for like() has to be a text");
 			static_assert(is_text_t<Pattern>::value, "Pattern for like() has to be a text");
diff --git a/include/sqlpp11/vendor/select_pseudo_table.h b/include/sqlpp11/vendor/select_pseudo_table.h
index e431b16a..196b2ae5 100644
--- a/include/sqlpp11/vendor/select_pseudo_table.h
+++ b/include/sqlpp11/vendor/select_pseudo_table.h
@@ -37,7 +37,11 @@ namespace sqlpp
 		{
 			using _name_t = typename Expr::_name_t;
 			using _value_type = typename Expr::_value_type;
-			struct _column_type {};
+			struct _column_type 
+			{
+        using _must_not_insert = std::true_type;
+        using _must_not_update = std::true_type;
+			};
 		};
 
 	template<
diff --git a/include/sqlpp11/vendor/wrap_operand.h b/include/sqlpp11/vendor/wrap_operand.h
index 4f6e11ac..7ec98785 100644
--- a/include/sqlpp11/vendor/wrap_operand.h
+++ b/include/sqlpp11/vendor/wrap_operand.h
@@ -227,6 +227,10 @@ namespace sqlpp
 			};
 
 		// FIXME: Need to allow std::ref arguments
+
+		template<typename T>
+			using wrap_operand_t = typename wrap_operand<T>::type;
+
 	}
 }
 
diff --git a/tests/FunctionTest.cpp b/tests/FunctionTest.cpp
index 1b37be29..98da9365 100644
--- a/tests/FunctionTest.cpp
+++ b/tests/FunctionTest.cpp
@@ -40,6 +40,8 @@ int main()
 	test::TabFoo f; 
 	test::TabBar t;
 
+	//f.omega + 4 *= "";
+
 	// MEMBER FUNCTIONS
 	// ----------------
 	
diff --git a/tests/SelectTypeTest.cpp b/tests/SelectTypeTest.cpp
index fbe41a62..748fa700 100644
--- a/tests/SelectTypeTest.cpp
+++ b/tests/SelectTypeTest.cpp
@@ -245,8 +245,8 @@ int main()
 		static_assert(sqlpp::is_expression_t<T>::value, "type requirement");
 		static_assert(sqlpp::is_named_expression_t<T>::value, "type requirement");
 		static_assert(not sqlpp::require_insert_t<T>::value, "type requirement");
-		static_assert(not sqlpp::must_not_insert_t<T>::value, "type requirement");
-		static_assert(not sqlpp::must_not_update_t<T>::value, "type requirement");
+		static_assert(sqlpp::must_not_insert_t<T>::value, "type requirement");
+		static_assert(sqlpp::must_not_update_t<T>::value, "type requirement");
 		static_assert(not sqlpp::is_boolean_t<T>::value, "type requirement");
 		static_assert(not sqlpp::is_text_t<T>::value, "type requirement");
 		static_assert(not sqlpp::is_alias_t<T>::value, "type requirement");
@@ -261,8 +261,8 @@ int main()
 		static_assert(sqlpp::is_expression_t<T>::value, "type requirement");
 		static_assert(sqlpp::is_named_expression_t<T>::value, "type requirement");
 		static_assert(not sqlpp::require_insert_t<T>::value, "type requirement");
-		static_assert(not sqlpp::must_not_insert_t<T>::value, "type requirement");
-		static_assert(not sqlpp::must_not_update_t<T>::value, "type requirement");
+		static_assert(sqlpp::must_not_insert_t<T>::value, "type requirement");
+		static_assert(sqlpp::must_not_update_t<T>::value, "type requirement");
 		static_assert(not sqlpp::is_boolean_t<T>::value, "type requirement");
 		static_assert(not sqlpp::is_text_t<T>::value, "type requirement");
 		static_assert(not sqlpp::is_alias_t<T>::value, "type requirement");