diff --git a/include/sqlpp11/select_column_list.h b/include/sqlpp11/select_column_list.h
index 76a61c76..727e7513 100644
--- a/include/sqlpp11/select_column_list.h
+++ b/include/sqlpp11/select_column_list.h
@@ -286,6 +286,7 @@ namespace sqlpp
 					template<typename AliasProvider>
 						_alias_t<AliasProvider> as(const AliasProvider& aliasProvider) const
 						{
+							run_check_t<_statement_t>::_();
 							static_assert(_statement_t::_can_be_used_as_table(), "statement cannot be used as table, e.g. due to missing tables");
 							static_assert(logic::none_t<is_multi_column_t<Columns>::value...>::value, "cannot use multi-columns in sub selects");
 							return _table_t<AliasProvider>(_get_statement()).as(aliasProvider);
diff --git a/include/sqlpp11/statement.h b/include/sqlpp11/statement.h
index 02aaca64..3b4f5b56 100644
--- a/include/sqlpp11/statement.h
+++ b/include/sqlpp11/statement.h
@@ -107,6 +107,9 @@ namespace sqlpp
 				template<typename Expression>
 					using _no_unknown_tables = detail::is_subset_of<required_tables_of<Expression>, _known_tables>;
 
+				template<template<typename> class Predicate>
+				using any_t = logic::any_t<Predicate<Policies>::value...>;
+
 				// The tables not covered by the from.
 				using _required_tables = detail::make_difference_set_t<
 					_all_required_tables,
@@ -130,7 +133,7 @@ namespace sqlpp
 				//   - the select is complete (leaks no tables)
 				static constexpr bool _can_be_used_as_table()
 				{
-					return is_select_column_list_t<_result_type_provider>::value
+					return has_result_row_t<_statement_t>::value
 					 	and _required_tables::size::value == 0
 					 	and _required_ctes::size::value == 0
 						? true
@@ -183,15 +186,14 @@ namespace sqlpp
 	{
 		using _policies_t = typename detail::statement_policies_t<Db, Policies...>;
 
+		using _consistency_check = detail::get_first_if<is_inconsistent_t, consistent_t, 
+					typename Policies::template _base_t<_policies_t>::_consistency_check..., 
+					typename _policies_t::_table_check,
+					typename _policies_t::_cte_check>;
 		using _run_check = detail::get_first_if<is_inconsistent_t, consistent_t, 
 					typename _policies_t::_parameter_check, 
-					typename Policies::template _base_t<_policies_t>::_consistency_check..., 
-					typename _policies_t::_table_check,
-					typename _policies_t::_cte_check>;
-		using _prepare_check = detail::get_first_if<is_inconsistent_t, consistent_t, 
-					typename Policies::template _base_t<_policies_t>::_consistency_check..., 
-					typename _policies_t::_table_check,
-					typename _policies_t::_cte_check>;
+					_consistency_check>;
+		using _prepare_check = _consistency_check;
 
 		using _result_type_provider = typename _policies_t::_result_type_provider;
 		template<typename Composite>
diff --git a/include/sqlpp11/type_traits.h b/include/sqlpp11/type_traits.h
index 4cf352ec..31f6a68e 100644
--- a/include/sqlpp11/type_traits.h
+++ b/include/sqlpp11/type_traits.h
@@ -339,6 +339,57 @@ namespace sqlpp
 	template<typename Context, typename T>
 		using serialize_check_t = typename serialize_check<Context, T>::type;
 
+	template<typename Statement, typename Enable = void>
+		struct has_result_row_impl
+		{
+			using type = std::false_type;
+		};
+
+	template<typename Statement>
+		struct has_result_row_impl<Statement, 
+		typename std::enable_if<
+			not wrong_t<typename Statement::template _result_methods_t<Statement>::template _result_row_t<void>>::value, 
+		void>::type>
+		{
+			using type = std::true_type;
+		};
+
+	template<typename Statement>
+		using has_result_row_t = typename has_result_row_impl<Statement>::type;
+
+	template<typename Statement, typename Enable = void>
+		struct get_result_row_impl
+		{
+			using type = void;
+		};
+
+	template<typename Statement>
+		struct get_result_row_impl<Statement, 
+		typename std::enable_if<
+			not wrong_t<typename Statement::template _result_methods_t<Statement>::template _result_row_t<void>>::value, 
+		void>::type>
+		{
+			using type = typename Statement::template _result_methods_t<Statement>::template _result_row_t<void>;
+		};
+
+	template<typename Statement>
+		using get_result_row_t = typename get_result_row_impl<Statement>::type;
+
+	template<typename Statement, template<typename> class Predicate, typename Enable=void>
+		struct has_policy_impl
+		{
+			using type = std::false_type;
+		};
+
+	template<typename Statement, template<typename> class Predicate>
+		struct has_policy_impl<Statement, Predicate, typename std::enable_if<is_statement_t<Statement>::value>::type>
+		{
+			using type = std::true_type;
+		};
+
+	template<typename Statement, template<typename> class Predicate>
+		using has_policy_t = typename has_policy_impl<Statement, Predicate>::type;
+
 }
 
 #endif
diff --git a/include/sqlpp11/union.h b/include/sqlpp11/union.h
index 56559421..d68ab63b 100644
--- a/include/sqlpp11/union.h
+++ b/include/sqlpp11/union.h
@@ -37,42 +37,6 @@
 
 namespace sqlpp
 {
-	template<typename Statement, typename Enable = void>
-		struct has_result_row_impl
-		{
-			using type = std::false_type;
-		};
-
-	template<typename Statement>
-		struct has_result_row_impl<Statement, 
-		typename std::enable_if<
-			not wrong_t<typename Statement::template _result_methods_t<Statement>::template _result_row_t<void>>::value, 
-		void>::type>
-		{
-			using type = std::true_type;
-		};
-
-	template<typename Statement>
-		using has_result_row_t = typename has_result_row_impl<Statement>::type;
-
-	template<typename Statement, typename Enable = void>
-		struct get_result_row_impl
-		{
-			using type = void;
-		};
-
-	template<typename Statement>
-		struct get_result_row_impl<Statement, 
-		typename std::enable_if<
-			not wrong_t<typename Statement::template _result_methods_t<Statement>::template _result_row_t<void>>::value, 
-		void>::type>
-		{
-			using type = typename Statement::template _result_methods_t<Statement>::template _result_row_t<void>;
-		};
-
-	template<typename Statement>
-		using get_result_row_t = typename get_result_row_impl<Statement>::type;
-
 
 	struct no_union_t;
 
@@ -119,7 +83,7 @@ namespace sqlpp
 	template<typename Database, typename Flag, typename Lhs, typename Rhs>
 		struct union_t
 		{
-			using _traits = make_traits<no_value_t, tag::is_union, tag::is_select_column_list, tag::is_return_value>;
+			using _traits = make_traits<no_value_t, tag::is_union, tag::is_return_value>;
 			using _recursive_traits = make_recursive_traits<Lhs, Rhs>;
 
 			using _alias_t = struct{};
@@ -145,9 +109,9 @@ namespace sqlpp
 					_impl_t<Policies>& operator()() { return union_; }
 					const _impl_t<Policies>& operator()() const { return union_; }
 
-					using _selected_columns_t = decltype(union_._data._lhs.selected_columns);
-					_selected_columns_t& get_selected_columns() { return union_._data._lhs.selected_columns;}
-					const _selected_columns_t& get_selected_columns() const { return union_._data._lhs.selected_columns; }
+					using _selected_columns_t = typename std::decay<decltype(union_._data._lhs.get_selected_columns())>::type;
+					_selected_columns_t& get_selected_columns() { return union_._data._lhs.get_selected_columns(); }
+					const _selected_columns_t& get_selected_columns() const { return union_._data._lhs.get_selected_columns(); }
 
 					template<typename T>
 						static auto _get_member(T t) -> decltype(t.union_)
@@ -155,7 +119,9 @@ namespace sqlpp
 							return t.union_;
 						}
 
-					using _consistency_check = consistent_t;
+					using _consistency_check = detail::get_first_if<is_inconsistent_t, consistent_t,
+								typename Lhs::_consistency_check,
+								typename Rhs::_consistency_check>;
 				};
 
 			template<typename Statement>
@@ -208,8 +174,8 @@ namespace sqlpp
 					auto union_distinct(Rhs rhs) const
 					-> _new_statement_t<_check<Rhs>, union_t<void, distinct_t, derived_statement_t<Policies>, Rhs>>
 					{
-#warning: make sure that Rhs is a select, not a uninon
 						static_assert(is_statement_t<Rhs>::value, "argument of union call has to be a statement");
+						static_assert(has_policy_t<Rhs, is_select_t>::value, "argument of union call has to be a select");
 						static_assert(has_result_row_t<Rhs>::value, "argument of a union has to be a (complete) select statement");
 						static_assert(has_result_row_t<derived_statement_t<Policies>>::value, "left hand side argument of a union has to be a (complete) select statement");
 						static_assert(std::is_same<get_result_row_t<derived_statement_t<Policies>>, get_result_row_t<Rhs>>::value, "both select statements in a union have to have the same result columns (type and name)");
diff --git a/tests/UnionTest.cpp b/tests/UnionTest.cpp
index b4462ded..ca88841c 100644
--- a/tests/UnionTest.cpp
+++ b/tests/UnionTest.cpp
@@ -37,12 +37,17 @@ int main()
 	test::TabBar t;
 	test::TabFoo f;
 
-	db(select(t.alpha).from(t).union_distinct(select(f.epsilon.as(t.alpha)).from(f)));
-	db(select(t.alpha).from(t).union_all(select(f.epsilon.as(t.alpha)).from(f)));
+	db(select(t.alpha).from(t).where(true)
+			.union_distinct(select(f.epsilon.as(t.alpha)).from(f).where(true)));
+	db(select(t.alpha).from(t).where(true)
+			.union_all(select(f.epsilon.as(t.alpha)).from(f).where(true)));
 
-	auto u = select(t.alpha).from(t).union_all(select(f.epsilon.as(t.alpha)).from(f)).as(sqlpp::alias::u);
+	auto u = select(t.alpha).from(t).where(true).union_all(select(f.epsilon.as(t.alpha)).from(f).where(true)).as(sqlpp::alias::u);
 
-	db(select(all_of(u)).from(u).union_all(select(t.delta.as(t.alpha)).from(t)));
+	db(select(all_of(u)).from(u).where(true).union_all(select(t.delta.as(t.alpha)).from(t).where(true)));
+	db(select(u.alpha).from(u).where(true).union_all(select(t.delta.as(t.alpha)).from(t).where(true)));
+
+	db(select(t.alpha).from(t).where(true).union_all(select(t.alpha).from(t).where(true)).union_all(select(t.alpha).from(t).where(true)));
 
 	return 0;
 }