Added a few reminders which expressions can be null as well

include/sqlpp11/functions.h

@@ -52,6 +52,7 @@ namespace sqlpp
 			return { t };
 		}
 
+#warning: verbatim_t always "can_be_null" (espcially since we cannot determine the required tables later on)
 	template<typename ValueType> // Csaba Csoma suggests: unsafe_sql instead of verbatim
 		struct verbatim_t: public ValueType::template expression_operators<verbatim_t<ValueType>>,
 		public alias_operators<verbatim_t<ValueType>>

include/sqlpp11/select_column_list.h

@@ -54,6 +54,7 @@ namespace sqlpp
 		template<typename Column>
 			struct select_traits<Column>
 			{
+#warning need to transport the "can be null information here"
 				using _traits = make_traits<value_type_of<Column>, tag::select_column_list, tag::return_value, tag::expression, tag::named_expression>;
 				using _name_t = typename Column::_name_t;
 			};

include/sqlpp11/verbatim_table.h

@@ -46,6 +46,7 @@ namespace sqlpp
 		};
 	}
 
+#warning a verbatim table is always an outer table
 	struct verbatim_table_t: public sqlpp::table_t<verbatim_table_t, detail::unusable_pseudo_column_t>
 	{
 		struct _name_t {};