diff --git a/include/sqlpp11/core/clause/from.h b/include/sqlpp11/core/clause/from.h
index a58ac405..890582d3 100644
--- a/include/sqlpp11/core/clause/from.h
+++ b/include/sqlpp11/core/clause/from.h
@@ -83,7 +83,7 @@ namespace sqlpp
     using type = static_combined_check_t<
         static_check_t<not is_pre_join_t<Table>::value, assert_from_not_pre_join_t>,
         static_check_t<is_table<Table>::value, assert_from_table_t>,
-        static_check_t<required_tables_of_t<Table>::size::value == 0, assert_from_dependency_free_t>
+        static_check_t<required_tables_of_t<Table>::empty(), assert_from_dependency_free_t>
         >;
   };
 
diff --git a/include/sqlpp11/core/clause/group_by.h b/include/sqlpp11/core/clause/group_by.h
index 6cabbf84..69a82591 100644
--- a/include/sqlpp11/core/clause/group_by.h
+++ b/include/sqlpp11/core/clause/group_by.h
@@ -75,7 +75,7 @@ namespace sqlpp
 
       _data_t _data;
 
-      using _consistency_check = typename std::conditional<Policies::template _no_unknown_tables<group_by_t>::value,
+      using _consistency_check = typename std::conditional<Policies::template _no_unknown_tables<group_by_t>,
                                                            consistent_t,
                                                            assert_no_unknown_tables_in_group_by_t>::type;
     };
diff --git a/include/sqlpp11/core/clause/having.h b/include/sqlpp11/core/clause/having.h
index 214799c4..0b682165 100644
--- a/include/sqlpp11/core/clause/having.h
+++ b/include/sqlpp11/core/clause/having.h
@@ -78,7 +78,7 @@ namespace sqlpp
 
       _data_t _data;
 
-      using _table_check = typename std::conditional<Policies::template _no_unknown_tables<having_t>::value,
+      using _table_check = typename std::conditional<Policies::template _no_unknown_tables<having_t>,
                                                      consistent_t,
                                                      assert_having_no_unknown_tables_t>::type;
 
diff --git a/include/sqlpp11/core/clause/insert_value_list.h b/include/sqlpp11/core/clause/insert_value_list.h
index 90831ec2..36e4348d 100644
--- a/include/sqlpp11/core/clause/insert_value_list.h
+++ b/include/sqlpp11/core/clause/insert_value_list.h
@@ -202,7 +202,7 @@ namespace sqlpp
 
       _data_t _data;
 
-      using _consistency_check = typename std::conditional<Policies::template _no_unknown_tables<insert_list_t>::value,
+      using _consistency_check = typename std::conditional<Policies::template _no_unknown_tables<insert_list_t>,
                                                            consistent_t,
                                                            assert_no_unknown_tables_in_insert_assignments_t>::type;
     };
@@ -282,7 +282,7 @@ namespace sqlpp
       void _add_impl(const std::false_type& /*unused*/, Assignments... /*unused*/);
 
     public:
-      using _consistency_check = typename std::conditional<Policies::template _no_unknown_tables<column_list_t>::value,
+      using _consistency_check = typename std::conditional<Policies::template _no_unknown_tables<column_list_t>,
                                                            consistent_t,
                                                            assert_no_unknown_tables_in_column_list_t>::type;
     };
diff --git a/include/sqlpp11/core/clause/order_by.h b/include/sqlpp11/core/clause/order_by.h
index a62e5107..33190dce 100644
--- a/include/sqlpp11/core/clause/order_by.h
+++ b/include/sqlpp11/core/clause/order_by.h
@@ -75,7 +75,7 @@ namespace sqlpp
 
       _data_t _data;
 
-      using _consistency_check = typename std::conditional<Policies::template _no_unknown_tables<order_by_t>::value,
+      using _consistency_check = typename std::conditional<Policies::template _no_unknown_tables<order_by_t>,
                                                            consistent_t,
                                                            assert_no_unknown_tables_in_order_by_t>::type;
     };
diff --git a/include/sqlpp11/core/clause/select_column_list.h b/include/sqlpp11/core/clause/select_column_list.h
index 6a5daf32..9f299312 100644
--- a/include/sqlpp11/core/clause/select_column_list.h
+++ b/include/sqlpp11/core/clause/select_column_list.h
@@ -132,7 +132,7 @@ namespace sqlpp
         return *this;
       }
 
-      using _table_check = typename std::conditional<Policies::template _no_unknown_tables<select_column_list_t>::value,
+      using _table_check = typename std::conditional<Policies::template _no_unknown_tables<select_column_list_t>,
                                                      consistent_t,
                                                      assert_no_unknown_tables_in_selected_columns_t>::type;
 
diff --git a/include/sqlpp11/core/clause/update_list.h b/include/sqlpp11/core/clause/update_list.h
index c8eb3a92..941488a8 100644
--- a/include/sqlpp11/core/clause/update_list.h
+++ b/include/sqlpp11/core/clause/update_list.h
@@ -73,7 +73,7 @@ namespace sqlpp
 
       _data_t _data;
 
-      using _consistency_check = typename std::conditional<Policies::template _no_unknown_tables<update_list_t>::value,
+      using _consistency_check = typename std::conditional<Policies::template _no_unknown_tables<update_list_t>,
                                                            consistent_t,
                                                            assert_no_unknown_tables_in_update_assignments_t>::type;
     };
diff --git a/include/sqlpp11/core/clause/where.h b/include/sqlpp11/core/clause/where.h
index 7b066b1f..f3f4826a 100644
--- a/include/sqlpp11/core/clause/where.h
+++ b/include/sqlpp11/core/clause/where.h
@@ -65,7 +65,7 @@ namespace sqlpp
 
       _data_t _data;
 
-      using _consistency_check = typename std::conditional<Policies::template _no_unknown_tables<where_t>::value,
+      using _consistency_check = typename std::conditional<Policies::template _no_unknown_tables<where_t>,
                                                            consistent_t,
                                                            assert_no_unknown_tables_in_where_t>::type;
     };
diff --git a/include/sqlpp11/core/detail/type_vector.h b/include/sqlpp11/core/detail/type_vector.h
index 4f699940..d492479f 100644
--- a/include/sqlpp11/core/detail/type_vector.h
+++ b/include/sqlpp11/core/detail/type_vector.h
@@ -48,6 +48,12 @@ namespace sqlpp
         return ::sqlpp::logic::all<contains<X>()...>::value;
       }
 
+      template <typename... X>
+      static constexpr bool contains_any(type_vector<X...>)
+      {
+        return ::sqlpp::logic::any<contains<X>()...>::value;
+      }
+
       template <typename... X>
       static constexpr bool contains_none(type_vector<X...>)
       {
diff --git a/include/sqlpp11/core/field_spec.h b/include/sqlpp11/core/field_spec.h
index cb03c69e..8502bd8e 100644
--- a/include/sqlpp11/core/field_spec.h
+++ b/include/sqlpp11/core/field_spec.h
@@ -77,13 +77,12 @@ namespace sqlpp
     struct make_field_spec_impl
     {
       using ValueType = select_column_value_type_of_t<NamedExpr>;
-      static constexpr bool _depends_on_outer_table =
-          detail::make_intersect_set_t<required_tables_of_t<NamedExpr>,
-                                       typename Select::_used_outer_tables>::size::value > 0;
+      static constexpr bool _depends_on_optional_table =
+          Select::_used_optional_tables::contains_any(required_tables_of_t<NamedExpr>{});
 
       using type = field_spec_t<
           select_column_name_tag_of_t<NamedExpr>,
-          typename std::conditional<_depends_on_outer_table, sqlpp::force_optional_t<ValueType>, ValueType>::type>;
+          typename std::conditional<_depends_on_optional_table, sqlpp::force_optional_t<ValueType>, ValueType>::type>;
     };
   }  // namespace detail
 
diff --git a/include/sqlpp11/core/operator/as_expression.h b/include/sqlpp11/core/operator/as_expression.h
index 8c8dad83..a50e4c87 100644
--- a/include/sqlpp11/core/operator/as_expression.h
+++ b/include/sqlpp11/core/operator/as_expression.h
@@ -29,6 +29,7 @@ SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #include <type_traits>
 
 #include <sqlpp11/core/type_traits.h>
+#include <sqlpp11/core/to_sql_string.h>
 
 namespace sqlpp
 {
diff --git a/include/sqlpp11/core/query/statement.h b/include/sqlpp11/core/query/statement.h
index 3af5b9d8..a74b2f2b 100644
--- a/include/sqlpp11/core/query/statement.h
+++ b/include/sqlpp11/core/query/statement.h
@@ -79,14 +79,11 @@ namespace sqlpp
       using _all_provided_ctes = detail::make_joined_set_t<provided_ctes_of<Policies>...>;
       using _all_required_tables = detail::type_vector_cat_t<required_tables_of_t<Policies>...>;
       using _all_provided_tables = detail::type_vector_cat_t<provided_tables_of_t<Policies>...>;
-      //using _all_provided_outer_tables = detail::make_joined_set_t<provided_optional_tables_of_t<Policies>...>;
+      using _all_provided_optional_tables = detail::type_vector_cat_t<provided_optional_tables_of_t<Policies>...>;
       using _all_provided_aggregates = detail::make_joined_set_t<provided_aggregates_of<Policies>...>;
 
-#warning reactivate
-      /*
       template <typename Expression>
-      using _no_unknown_tables = detail::is_subset_of<required_tables_of_t<Expression>, _all_provided_tables>;
-      */
+      static constexpr bool _no_unknown_tables = _all_provided_tables::contains_all(required_tables_of_t<Expression>{});
 
       // workaround for msvc bug https://connect.microsoft.com/VisualStudio/Feedback/Details/2086629
       //	  template <typename... Expressions>
@@ -144,7 +141,7 @@ namespace sqlpp
       /*
       using _can_be_null = logic::any<can_be_null_t<_result_type_provider>::value,
                                         detail::make_intersect_set_t<required_tables_of_t<_result_type_provider>,
-                                                                     _all_provided_outer_tables>::size::value != 0>;
+                                                                     _all_provided_optional_tables>::size::value != 0>;
                                         */
       using _parameters = detail::type_vector_cat_t<parameters_of<Policies>...>;
       // required_tables and _required_ctes are defined above
@@ -211,7 +208,8 @@ namespace sqlpp
                       >;
     using _name_tag_of = name_tag_of<_result_type_provider>;
     using _nodes = detail::type_vector<_policies_t>;
-    //using _used_outer_tables = typename _policies_t::_all_provided_outer_tables;
+#warning: This is an odd name, why "used"?
+    using _used_optional_tables = typename _policies_t::_all_provided_optional_tables;
 
     // Constructors
     statement_t() = default;
diff --git a/include/sqlpp11/postgresql/returning_column_list.h b/include/sqlpp11/postgresql/returning_column_list.h
index ad2ff468..f524366f 100644
--- a/include/sqlpp11/postgresql/returning_column_list.h
+++ b/include/sqlpp11/postgresql/returning_column_list.h
@@ -112,7 +112,7 @@ namespace sqlpp
 
         // Checks
         using _table_check =
-            typename std::conditional<Policies::template _no_unknown_tables<returning_column_list_t>::value,
+            typename std::conditional<Policies::template _no_unknown_tables<returning_column_list_t>,
                                       consistent_t,
                                       assert_no_unknown_tables_in_returning_columns_t>::type;
 
diff --git a/tests/core/serialize/operator/as_expression.cpp b/tests/core/serialize/operator/as_expression.cpp
index 7330915f..a2000021 100644
--- a/tests/core/serialize/operator/as_expression.cpp
+++ b/tests/core/serialize/operator/as_expression.cpp
@@ -43,12 +43,12 @@ int main(int, char* [])
   SQLPP_COMPARE(dynamic(true, val).as(v), "17 AS v");
   SQLPP_COMPARE(dynamic(true, expr).as(v), "(17 + 4) AS v");
   SQLPP_COMPARE(dynamic(true, count(val)).as(v), "COUNT(17) AS v");
-  SQLPP_COMPARE(dynamic(true, col.id).as(v), "tab_foo.id AS v");
+  SQLPP_COMPARE(dynamic(true, col_id).as(v), "tab_foo.id AS v");
 
   SQLPP_COMPARE(dynamic(false, val).as(v), "NULL AS v");
   SQLPP_COMPARE(dynamic(false, expr).as(v), "NULL AS v");
   SQLPP_COMPARE(dynamic(false, count(val)).as(v), "NULL AS v");
-  SQLPP_COMPARE(dynamic(false, col.id).as(v), "NULL AS v");
+  SQLPP_COMPARE(dynamic(false, col_id).as(v), "NULL AS v");
 
   return 0;
 }