Allow alias of single-column select to be used as select column (#542)

include/sqlpp11/select_pseudo_table.h

@@ -50,11 +50,31 @@ namespace sqlpp
                                 tag_if<tag::can_be_null, _can_be_null or _depends_on_outer_table>>;
   };
 
+  template<typename... NamedExpr>
+  struct select_expression_type {
+    using value_t = no_value_t;
+    static constexpr bool _is_expression = false;
+    static constexpr bool _can_be_null = false;
+  };
+
+  template<typename NamedExpr>
+  struct select_expression_type<NamedExpr> {
+    using value_t = value_type_of<NamedExpr>;
+    static constexpr bool _is_expression = true;
+    static constexpr bool _can_be_null = can_be_null_t<NamedExpr>::value;
+  };
+
   template <typename Select, typename... NamedExpr>
   struct select_pseudo_table_t
       : public table_t<select_pseudo_table_t<Select, NamedExpr...>, select_column_spec_t<Select, NamedExpr>...>
   {
-    using _traits = make_traits<no_value_t,
+    using _expr_t = select_expression_type<NamedExpr...>;
+    using _traits = make_traits<
+        // Usage as named expression
+        typename _expr_t::value_t,
+        tag_if<tag::is_expression, _expr_t::_is_expression>,
+        tag_if<tag::can_be_null, _expr_t::_can_be_null>,
+        // Usage as table
         tag::is_table,
         tag::is_pseudo_table,
         tag_if<tag::requires_parens, requires_parens_t<Select>::value>>;

include/sqlpp11/table_alias.h

@@ -44,6 +44,7 @@ namespace sqlpp
     using _traits = make_traits<value_type_of<Table>,
                                 tag::is_table,
                                 tag::is_alias,
+                                tag_if<tag::can_be_null, can_be_null_t<Table>::value>,
                                 tag_if<tag::is_selectable, is_expression_t<Table>::value>>;
 
     using _nodes = detail::type_vector<>;

tests/core/serialize/CMakeLists.txt

@@ -43,6 +43,7 @@ set(test_files
     Min.cpp
     Operator.cpp
     Over.cpp
+    SelectAs.cpp
     Some.cpp
     Sum.cpp
     TableAlias.cpp

tests/core/serialize/SelectAs.cpp

@@ -0,0 +1,43 @@
+/*
+ * Copyright (c) 2023, 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.
+ */
+
+#include "compare.h"
+#include "Sample.h"
+#include <sqlpp11/sqlpp11.h>
+
+#include <iostream>
+
+SQLPP_ALIAS_PROVIDER(cheese)
+
+int SelectAs(int, char*[])
+{
+  const auto foo = test::TabFoo{};
+  const auto bar = test::TabBar{};
+
+  // SELECT...AS as selectable column
+  compare(__LINE__, select(foo.omega, select(count(bar.alpha)).from(bar).unconditionally().as(cheese)), "SELECT tab_foo.omega,(SELECT COUNT(tab_bar.alpha) AS count_ FROM tab_bar) AS cheese");
+
+  return 0;
+}

tests/core/usage/Select.cpp

@@ -61,6 +61,8 @@ void print_row(Row const& row)
   std::cout << a << ", " << b << std::endl;
 }
 
+SQLPP_ALIAS_PROVIDER(cheese)
+
 int Select(int, char*[])
 {
   MockDb db = {};
@@ -164,6 +166,7 @@ int Select(int, char*[])
                .dynamic_offset();
   s.select_flags.add(sqlpp::distinct);
   s.selected_columns.add(without_table_check(f.omega));
+  s.selected_columns.add(select(f.omega).from(f).unconditionally().as(f.delta));
   s.from.add(dynamic_cross_join(f));
   s.where.add(t.alpha > 7);
   s.having.add(t.alpha > 7);
@@ -210,5 +213,11 @@ int Select(int, char*[])
     }
   }
 
+  for (const auto& row :
+       db(select(f.omega, select(count(t.alpha)).from(t).unconditionally().as(cheese)).from(f).unconditionally()))
+  {
+    std::cout << row.omega << " " << row.cheese << std::endl;
+  }
+
   return 0;
 }

tests/core/usage/SelectType.cpp

@@ -260,9 +260,9 @@ int SelectType(int, char*[])
   // Test an alias of a select of a single numeric table column
   {
     using T = decltype(select(t.alpha).from(t).as(alias::b));
-    static_assert(not sqlpp::is_numeric_t<T>::value, "type requirement");
+    static_assert(sqlpp::is_numeric_t<T>::value, "type requirement");
     static_assert(not sqlpp::is_expression_t<T>::value, "type requirement");
-    static_assert(not sqlpp::is_selectable_t<T>::value, "type requirement");
+    static_assert(sqlpp::is_selectable_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");
@@ -276,9 +276,9 @@ int SelectType(int, char*[])
   // Test the column of an alias of a select of an alias of a single numeric table column
   {
     using T = decltype(select(t.alpha.as(alias::a)).from(t).as(alias::b));
-    static_assert(not sqlpp::is_numeric_t<T>::value, "type requirement");
+    static_assert(sqlpp::is_numeric_t<T>::value, "type requirement");
     static_assert(not sqlpp::is_expression_t<T>::value, "type requirement");
-    static_assert(not sqlpp::is_selectable_t<T>::value, "type requirement");
+    static_assert(sqlpp::is_selectable_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");