From fc0632e0b0e2cc900c519a0b1b7c73eba4406ec5 Mon Sep 17 00:00:00 2001
From: Roland Bock <rbock@eudoxos.de>
Date: Fri, 16 Aug 2024 14:15:08 +0200
Subject: [PATCH] more tests and more clarity for group_by

---
 include/sqlpp11/core/basic/column.h           |   5 +
 include/sqlpp11/core/clause/group_by.h        |  64 ++---
 .../sqlpp11/core/clause/select_column_list.h  |   1 +
 include/sqlpp11/core/group_by_column.h        |  73 +++++
 include/sqlpp11/core/query/statement.h        |   6 +
 include/sqlpp11/core/type_traits.h            | 253 +-----------------
 include/sqlpp11/core/type_traits/aggregates.h |  14 +-
 .../core/type_traits/group_by_column.h        |  46 ++++
 include/sqlpp11/core/type_traits/optional.h   |  79 ++++++
 include/sqlpp11/core/type_traits/value_type.h | 237 ++++++++++++++++
 tests/core/types/CMakeLists.txt               |   1 +
 tests/core/types/clause/CMakeLists.txt        |  33 +++
 tests/core/types/clause/group_by.cpp          |  68 +++++
 tests/core/types/clause/select_columns.cpp    |  53 ++++
 tests/core/types/type_traits/CMakeLists.txt   |   2 +-
 15 files changed, 647 insertions(+), 288 deletions(-)
 create mode 100644 include/sqlpp11/core/group_by_column.h
 create mode 100644 include/sqlpp11/core/type_traits/group_by_column.h
 create mode 100644 include/sqlpp11/core/type_traits/optional.h
 create mode 100644 include/sqlpp11/core/type_traits/value_type.h
 create mode 100644 tests/core/types/clause/CMakeLists.txt
 create mode 100644 tests/core/types/clause/group_by.cpp
 create mode 100644 tests/core/types/clause/select_columns.cpp

diff --git a/include/sqlpp11/core/basic/column.h b/include/sqlpp11/core/basic/column.h
index 626663d8..6f9f2463 100644
--- a/include/sqlpp11/core/basic/column.h
+++ b/include/sqlpp11/core/basic/column.h
@@ -85,6 +85,11 @@ namespace sqlpp
     }
   };
 
+  template<typename Table, typename ColumnSpec>
+  struct is_group_by_column<column_t<Table, ColumnSpec>> : public std::true_type
+  {
+  };
+
   template<typename Table, typename ColumnSpec>
   struct value_type_of<column_t<Table, ColumnSpec>>
   {
diff --git a/include/sqlpp11/core/clause/group_by.h b/include/sqlpp11/core/clause/group_by.h
index 221e3cf7..de96ed34 100644
--- a/include/sqlpp11/core/clause/group_by.h
+++ b/include/sqlpp11/core/clause/group_by.h
@@ -30,15 +30,16 @@
 #include <sqlpp11/core/logic.h>
 #include <sqlpp11/core/query/policy_update.h>
 #include <sqlpp11/core/type_traits.h>
+#include <sqlpp11/core/group_by_column.h>
 #include <tuple>
 
 namespace sqlpp
 {
   // GROUP BY DATA
-  template <typename... Expressions>
+  template <typename... Columns>
   struct group_by_data_t
   {
-    group_by_data_t(Expressions... expressions) : _expressions(expressions...)
+    group_by_data_t(Columns... columns) : _columns(columns...)
     {
     }
 
@@ -48,7 +49,7 @@ namespace sqlpp
     group_by_data_t& operator=(group_by_data_t&&) = default;
     ~group_by_data_t() = default;
 
-    std::tuple<Expressions...> _expressions;
+    std::tuple<Columns...> _columns;
   };
 
   SQLPP_PORTABLE_STATIC_ASSERT(
@@ -56,13 +57,13 @@ namespace sqlpp
       "at least one group-by expression requires a table which is otherwise not known in the statement");
 
   // GROUP BY
-  template <typename... Expressions>
+  template <typename... Columns>
   struct group_by_t
   {
     using _traits = make_traits<no_value_t, tag::is_group_by>;
-    using _nodes = detail::type_vector<Expressions...>;
+    using _nodes = detail::type_vector<Columns...>;
 
-   using _data_t = group_by_data_t<Expressions...>;
+   using _data_t = group_by_data_t<Columns...>;
 
     // Base template to be inherited by the statement
     template <typename Policies>
@@ -80,22 +81,22 @@ namespace sqlpp
     };
   };
 
-  template <typename... Expressions>
-  struct known_aggregate_expressions_of<group_by_t<Expressions...>>
+  template <typename... Columns>
+  struct known_aggregate_columns_of<group_by_t<Columns...>>
   {
-    using type = detail::type_vector<Expressions...>;
+    using type = detail::type_vector<Columns...>;
   };
 
-  SQLPP_PORTABLE_STATIC_ASSERT(assert_group_by_args_have_values_t, "all arguments for group_by() must have values");
+  SQLPP_PORTABLE_STATIC_ASSERT(assert_group_by_args_are_columns_t, "all arguments for group_by() must be columns");
 
-  template <typename... Exprs>
+  template <typename... Columns>
   struct check_group_by
   {
     using type = static_combined_check_t<
-        static_check_t<logic::all_t<has_value_type<Exprs>::value...>::value, assert_group_by_args_have_values_t>>;
+        static_check_t<logic::all_t<is_group_by_column<Columns>::value...>::value, assert_group_by_args_are_columns_t>>;
   };
-  template <typename... Exprs>
-  using check_group_by_t = typename check_group_by<Exprs...>::type;
+  template <typename... Columns>
+  using check_group_by_t = typename check_group_by<Columns...>::type;
 
   // NO GROUP BY YET
   struct no_group_by_t
@@ -122,44 +123,43 @@ namespace sqlpp
 
       using _consistency_check = consistent_t;
 
-#warning: Arguments should not be constants and not contain aggregate functions
-      template <typename... Expressions>
-      auto group_by(Expressions... expressions) const
-          -> _new_statement_t<check_group_by_t<Expressions...>, group_by_t<Expressions...>>
+      template <typename... Columns>
+      auto group_by(Columns... columns) const
+          -> _new_statement_t<check_group_by_t<remove_dynamic_t<Columns>...>, group_by_t<Columns...>>
       {
-        static_assert(sizeof...(Expressions), "at least one expression (e.g. a column) required in group_by()");
+        static_assert(sizeof...(Columns), "at least one column required in group_by()");
 
-        return _group_by_impl(check_group_by_t<Expressions...>{}, expressions...);
+        return _group_by_impl(check_group_by_t<Columns...>{}, columns...);
       }
 
     private:
-      template <typename Check, typename... Expressions>
-      auto _group_by_impl(Check, Expressions... expressions) const -> inconsistent<Check>;
+      template <typename Check, typename... Columns>
+      auto _group_by_impl(Check, Columns... columns) const -> inconsistent<Check>;
 
-      template <typename... Expressions>
-      auto _group_by_impl(consistent_t /*unused*/, Expressions... expressions) const
-          -> _new_statement_t<consistent_t, group_by_t<Expressions...>>
+      template <typename... Columns>
+      auto _group_by_impl(consistent_t /*unused*/, Columns... columns) const
+          -> _new_statement_t<consistent_t, group_by_t<Columns...>>
       {
-        static_assert(not detail::has_duplicates<Expressions...>::value,
+        static_assert(not detail::has_duplicates<Columns...>::value,
                       "at least one duplicate argument detected in group_by()");
 
         return {static_cast<const derived_statement_t<Policies>&>(*this),
-                group_by_data_t<Expressions...>{expressions...}};
+                group_by_data_t<Columns...>{columns...}};
       }
     };
   };
 
   // Interpreters
-  template <typename Context, typename... Expressions>
-  auto to_sql_string(Context& context, const group_by_data_t<Expressions...>& t) -> std::string
+  template <typename Context, typename... Columns>
+  auto to_sql_string(Context& context, const group_by_data_t<Columns...>& t) -> std::string
   {
-    return " GROUP BY " + interpret_tuple(t._expressions, ',', context);
+    return " GROUP BY " + interpret_tuple(t._columns, ',', context);
   }
 
   template <typename... T>
-  auto group_by(T&&... t) -> decltype(statement_t<no_group_by_t>().group_by(std::forward<T>(t)...))
+  auto group_by(T... t) -> decltype(statement_t<no_group_by_t>().group_by(std::move(t)...))
   {
-    return statement_t<no_group_by_t>().group_by(std::forward<T>(t)...);
+    return statement_t<no_group_by_t>().group_by(std::move(t)...);
   }
 
 }  // namespace sqlpp
diff --git a/include/sqlpp11/core/clause/select_column_list.h b/include/sqlpp11/core/clause/select_column_list.h
index 32b8f5ec..6f10a0ad 100644
--- a/include/sqlpp11/core/clause/select_column_list.h
+++ b/include/sqlpp11/core/clause/select_column_list.h
@@ -35,6 +35,7 @@
 #include <sqlpp11/core/query/result_row.h>
 #include <sqlpp11/core/clause/select_as.h>
 #include <sqlpp11/core/clause/select_column_traits.h>
+#include <sqlpp11/core/group_by_column.h>
 #include <sqlpp11/core/basic/table.h>
 #include <tuple>
 
diff --git a/include/sqlpp11/core/group_by_column.h b/include/sqlpp11/core/group_by_column.h
new file mode 100644
index 00000000..efc48f53
--- /dev/null
+++ b/include/sqlpp11/core/group_by_column.h
@@ -0,0 +1,73 @@
+#pragma once
+
+/*
+ * Copyright (c) 2024, 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 <sqlpp11/core/type_traits/group_by_column.h>
+#include <sqlpp11/core/type_traits/value_type.h>
+#include <sqlpp11/core/operator/enable_as.h>
+
+namespace sqlpp
+{
+  // `group_by` takes columns as parameters.
+  // If you really, really want to, you can pass something else, but you have to tell the library that you are really certain by wrapping it in `declare_group_by_column`.
+
+  template <typename Expr>
+    struct group_by_column : public enable_as<group_by_column<Expr>>
+    {
+      group_by_column(Expr expr) : _expr(std::move(expr)) {}
+      group_by_column(const group_by_column&) = default;
+      group_by_column(group_by_column&&) = default;
+      group_by_column& operator=(const group_by_column&) = default;
+      group_by_column& operator=(group_by_column&&) = default;
+      ~group_by_column() = default;
+
+      Expr _expr;
+    };
+
+  template <typename Expr>
+  struct is_group_by_column<group_by_column<Expr>> : public std::true_type
+  {
+  };
+
+  template <typename Expr>
+  struct value_type_of<group_by_column<Expr>> : public value_type_of<Expr>
+  {
+  };
+
+  template<typename Expr>
+    using check_declare_group_by_column_args = ::sqlpp::enable_if_t<has_value_type<Expr>::value and not is_group_by_column<Expr>::value>;
+
+template <typename Expr, typename = check_declare_group_by_column_args<Expr>>
+  auto declare_group_by_column(Expr expr) -> group_by_column<Expr>
+  {
+    return {std::move(expr)};
+  }
+
+#warning Add tests
+
+}  // namespace sqlpp11
+
diff --git a/include/sqlpp11/core/query/statement.h b/include/sqlpp11/core/query/statement.h
index 102cfac6..7725a5b6 100644
--- a/include/sqlpp11/core/query/statement.h
+++ b/include/sqlpp11/core/query/statement.h
@@ -273,6 +273,12 @@ namespace sqlpp
     using type = typename detail::type_vector<Policies...>;
   };
 
+  template <typename... Policies>
+    struct known_aggregate_columns_of<statement_t<Policies...>>
+    {
+      using type = detail::type_vector_cat_t<known_aggregate_columns_of_t<Policies>...>;
+    };
+
   template <typename... Policies>
   struct requires_parentheses<statement_t<Policies...>> : public std::true_type {};
 
diff --git a/include/sqlpp11/core/type_traits.h b/include/sqlpp11/core/type_traits.h
index 1f8eae67..b3abcec6 100644
--- a/include/sqlpp11/core/type_traits.h
+++ b/include/sqlpp11/core/type_traits.h
@@ -1,7 +1,7 @@
 #pragma once
 
 /*
- * Copyright (c) 2013-2017, Roland Bock, Aaron Bishop
+ * Copyright (c) 2013, Roland Bock
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without modification,
@@ -26,13 +26,11 @@
  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-#include <cstdint>
 #include <tuple>
 #include <type_traits>
 #include <array>
 #include <vector>
 
-#include <sqlpp11/core/chrono.h>
 #include <sqlpp11/core/name/name_tag.h>
 #include <sqlpp11/core/compat/optional.h>
 #include <sqlpp11/core/compat/string_view.h>
@@ -44,55 +42,13 @@
 #include <sqlpp11/core/detail/type_set.h>
 #include <sqlpp11/core/detail/get_first.h>
 #include <sqlpp11/core/type_traits/aggregates.h>
+#include <sqlpp11/core/type_traits/group_by_column.h>
 #include <sqlpp11/core/type_traits/nodes_of.h>
+#include <sqlpp11/core/type_traits/optional.h>
+#include <sqlpp11/core/type_traits/value_type.h>
 
 namespace sqlpp
 {
-  template <typename T>
-  struct is_optional : public std::false_type
-  {
-  };
-
-  template <typename T>
-  struct is_optional<::sqlpp::optional<T>> : public std::true_type
-  {
-  };
-
-  template <>
-  struct is_optional<::sqlpp::nullopt_t> : public std::true_type
-  {
-  };
-
-  template <typename T>
-  struct remove_optional
-  {
-    using type = T;
-  };
-
-  template <typename T>
-  struct remove_optional<::sqlpp::optional<T>>
-  {
-    using type = T;
-  };
-
-  template <typename T>
-  using remove_optional_t = typename remove_optional<T>::type;
-
-  template <typename T>
-  struct force_optional
-  {
-    using type = ::sqlpp::optional<T>;
-  };
-
-  template <typename T>
-  struct force_optional<::sqlpp::optional<T>>
-  {
-    using type = ::sqlpp::optional<T>;
-  };
-
-  template <typename T>
-  using force_optional_t = typename force_optional<T>::type;
-
   template <typename T>
   const T& get_value(const T& t)
   {
@@ -117,26 +73,6 @@ namespace sqlpp
     return t.has_value();
   }
 
-  struct no_value_t;
-
-  template <typename T>
-  struct value_type_of
-  {
-    using type = no_value_t;
-  };
-
-  template <typename T>
-  using value_type_of_t = typename value_type_of<T>::type;
-
-  template<typename T>
-  struct value_type_of<::sqlpp::optional<T>>
-  {
-    using type = sqlpp::force_optional_t<value_type_of_t<remove_optional_t<T>>>;
-  };
-
-  template <typename T>
-  struct has_value_type : public std::integral_constant<bool, not std::is_same<value_type_of_t<T>, no_value_t>::value> {};
-
   template <typename T>
   struct has_default : public std::false_type
   {
@@ -153,186 +89,6 @@ namespace sqlpp
   };
 
   //
-  struct boolean{};
-  template<>
-  struct value_type_of<boolean> { using type = boolean; };
-  template<>
-  struct value_type_of<bool> { using type = boolean; };
-
-  struct integral{};
-  template<>
-  struct value_type_of<integral> { using type = integral; };
-  template<>
-  struct value_type_of<int8_t> { using type = integral; };
-  template<>
-  struct value_type_of<int16_t> { using type = integral; };
-  template<>
-  struct value_type_of<int32_t> { using type = integral; };
-  template<>
-  struct value_type_of<int64_t> { using type = integral; };
-
-  struct unsigned_integral{};
-  template<>
-  struct value_type_of<unsigned_integral> { using type = unsigned_integral; };
-  template<>
-  struct value_type_of<uint8_t> { using type = unsigned_integral; };
-  template<>
-  struct value_type_of<uint16_t> { using type = unsigned_integral; };
-  template<>
-  struct value_type_of<uint32_t> { using type = unsigned_integral; };
-  template<>
-  struct value_type_of<uint64_t> { using type = unsigned_integral; };
-
-  struct floating_point{};
-  template<>
-  struct value_type_of<floating_point> { using type = floating_point; };
-  template<>
-  struct value_type_of<float> { using type = floating_point; };
-  template<>
-  struct value_type_of<double> { using type = floating_point; };
-  template<>
-  struct value_type_of<long double> { using type = floating_point; };
-
-  struct text{};
-  template <>
-  struct value_type_of<text> { using type = text; };
-  template <>
-  struct value_type_of<char> { using type = text; };
-  template <>
-  struct value_type_of<const char*> { using type = text; };
-  template <>
-  struct value_type_of<std::string> { using type = text; };
-  template <>
-  struct value_type_of<::sqlpp::string_view> { using type = text; };
-
-  struct blob{};
-  template <>
-  struct value_type_of<blob> { using type = blob; };
-  template <std::size_t N>
-  struct value_type_of<std::array<std::uint8_t, N>> { using type = blob; };
-  template <>
-  struct value_type_of<std::vector<std::uint8_t>> { using type = blob; };
-  template <>
-  struct value_type_of<::sqlpp::span<std::uint8_t>> { using type = blob; };
-
-  struct day_point{};
-  template <>
-  struct value_type_of<day_point> { using type = day_point; };
-  template <>
-  struct value_type_of<std::chrono::time_point<std::chrono::system_clock, sqlpp::chrono::days>> { using type = day_point; };
-
-  struct time_of_day{};
-  template <>
-  struct value_type_of<time_of_day> { using type = time_of_day; };
-  template <typename Rep, typename Period>
-  struct value_type_of<std::chrono::duration<Rep, Period>> { using type = time_of_day; };
-
-  struct time_point{};
-  template <>
-  struct value_type_of<time_point> { using type = time_point; };
-  template <typename Period>
-  struct value_type_of<std::chrono::time_point<std::chrono::system_clock, Period>> { using type = time_point; };
-
-
-  template <typename T>
-  struct is_boolean : public std::is_same<remove_optional_t<value_type_of_t<T>>, boolean>
-  {
-  };
-
-  template <>
-  struct is_boolean<::sqlpp::nullopt_t> : public std::true_type {
-  };
-
-  template <typename T>
-  struct is_integral : public std::is_same<remove_optional_t<value_type_of_t<T>>, integral>
-  {
-  };
-
-  template <>
-  struct is_integral<::sqlpp::nullopt_t> : public std::true_type {
-  };
-
-  template <typename T>
-  struct is_unsigned_integral : public std::is_same<remove_optional_t<value_type_of_t<T>>, unsigned_integral>
-  {
-  };
-
-  template <>
-  struct is_unsigned_integral<::sqlpp::nullopt_t> : public std::true_type {
-  };
-
-  template <typename T>
-  struct is_floating_point : public std::is_same<remove_optional_t<value_type_of_t<T>>, floating_point>
-  {
-  };
-
-  template <>
-  struct is_floating_point<::sqlpp::nullopt_t> : public std::true_type {
-  };
-
-  // A generic numeric type which could be (unsigned) integral or floating point.
-  struct numeric{};
-  template <typename T>
-  struct is_numeric
-      : public std::integral_constant<bool,
-                                      is_boolean<T>::value or is_integral<T>::value or is_unsigned_integral<T>::value or
-                                          is_floating_point<T>::value or
-                                          std::is_same<remove_optional_t<value_type_of_t<T>>, numeric>::value>
-  {
-  };
-
-  template <>
-  struct is_numeric<::sqlpp::nullopt_t> : public std::true_type{};
-
-  template <typename T>
-  struct is_text : public std::is_same<remove_optional_t<value_type_of_t<T>>, text>
-  {
-  };
-
-  template <>
-  struct is_text<::sqlpp::nullopt_t> : public std::true_type {
-  };
-
-  template <typename T>
-  struct is_blob : public std::is_same<remove_optional_t<value_type_of_t<T>>, blob>
-  {
-  };
-
-  template <>
-  struct is_blob<::sqlpp::nullopt_t> : public std::true_type {
-  };
-
-  template <typename T>
-  struct is_day_point : public std::is_same<remove_optional_t<value_type_of_t<T>>, day_point>
-  {
-  };
-
-  template <>
-  struct is_day_point<::sqlpp::nullopt_t> : public std::true_type {
-  };
-
-  template <typename T>
-  struct is_time_point : public std::is_same<remove_optional_t<value_type_of_t<T>>, time_point>
-  {
-  };
-
-  template <>
-  struct is_time_point<::sqlpp::nullopt_t> : public std::true_type {
-  };
-
-  template <typename T>
-  struct is_day_or_time_point : public std::integral_constant<bool, is_day_point<T>::value or is_time_point<T>::value> {
-  };
-
-  template <typename T>
-  struct is_time_of_day : public std::is_same<remove_optional_t<value_type_of_t<T>>, time_of_day>
-  {
-  };
-
-  template <>
-  struct is_time_of_day<::sqlpp::nullopt_t> : public std::true_type {
-  };
-
   template <typename L, typename R>
   struct values_are_comparable
       : public std::integral_constant<bool,
@@ -579,6 +335,7 @@ namespace sqlpp
         ;
   };
 
+#warning: This should go away
   namespace detail
   {
     template <typename KnownAggregates, typename T, typename Leaf = void>
diff --git a/include/sqlpp11/core/type_traits/aggregates.h b/include/sqlpp11/core/type_traits/aggregates.h
index 6804baae..f0340d43 100644
--- a/include/sqlpp11/core/type_traits/aggregates.h
+++ b/include/sqlpp11/core/type_traits/aggregates.h
@@ -34,7 +34,7 @@ namespace sqlpp
 {
   // We don't want to mix aggregate and non-aggregate expressions as the results are unspecified.
   // Aggregates are either results of aggregate functions or GROUP BY columns.
-  // Non-aggregates are columns (unless they are aggregate expressions).
+  // Non-aggregates are columns (unless they are aggregate columns).
   // Constant values are neutral.
 
   template <typename T>
@@ -60,23 +60,23 @@ namespace sqlpp
   {
   };
 
-  // Obtain known aggregate expressions, i.e. GROUP BY columns.
+  // Obtain known aggregate columns, i.e. GROUP BY columns.
   template <typename T>
-  struct known_aggregate_expressions_of
+  struct known_aggregate_columns_of
   {
     using type = detail::type_vector<>;
   };
 
   template <typename T>
-  using known_aggregate_expressions_of_t = typename known_aggregate_expressions_of<T>::type;
+  using known_aggregate_columns_of_t = typename known_aggregate_columns_of<T>::type;
 
   // Checks if T is an aggregate expression (either an aggregate function or a known aggregate).
-  // @KnownAggregateExpressions: type_vector as obtained through known_aggregate_expressions_of_t
-  template <typename KnownAggregateExpressions, typename T>
+  // @KnownAggregateColumns: type_vector as obtained through known_aggregate_columns_of_t
+  template <typename KnownAggregateColumns, typename T>
   struct is_aggregate_expression
       : public std::integral_constant<bool,
                                       is_aggregate_function<T>::value or
-                                          KnownAggregateExpressions::template contains<T>::value>
+                                          KnownAggregateColumns::template contains<T>::value>
   {
   };
 
diff --git a/include/sqlpp11/core/type_traits/group_by_column.h b/include/sqlpp11/core/type_traits/group_by_column.h
new file mode 100644
index 00000000..2bd57542
--- /dev/null
+++ b/include/sqlpp11/core/type_traits/group_by_column.h
@@ -0,0 +1,46 @@
+#pragma once
+
+/*
+ * Copyright (c) 2024, 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 <sqlpp11/core/detail/type_vector.h>
+#include <sqlpp11/core/type_traits/value_type.h>
+#include <sqlpp11/core/logic.h>
+
+namespace sqlpp
+{
+  // `group_by` takes columns as parameters.
+  // If you really, really want to, you can pass something else, but you have to tell the library that you are really certain by wrapping it in `declare_group_by_column`.
+
+  template <typename T>
+  struct is_group_by_column : public std::false_type
+  {
+  };
+
+#warning Add tests
+
+}  // namespace sqlpp11
+
diff --git a/include/sqlpp11/core/type_traits/optional.h b/include/sqlpp11/core/type_traits/optional.h
new file mode 100644
index 00000000..b35662c5
--- /dev/null
+++ b/include/sqlpp11/core/type_traits/optional.h
@@ -0,0 +1,79 @@
+#pragma once
+
+/*
+ * Copyright (c) 2024, 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 <sqlpp11/core/compat/optional.h>
+
+namespace sqlpp
+{
+  template <typename T>
+  struct is_optional : public std::false_type
+  {
+  };
+
+  template <typename T>
+  struct is_optional<::sqlpp::optional<T>> : public std::true_type
+  {
+  };
+
+  template <>
+  struct is_optional<::sqlpp::nullopt_t> : public std::true_type
+  {
+  };
+
+  template <typename T>
+  struct remove_optional
+  {
+    using type = T;
+  };
+
+  template <typename T>
+  struct remove_optional<::sqlpp::optional<T>>
+  {
+    using type = T;
+  };
+
+  template <typename T>
+  using remove_optional_t = typename remove_optional<T>::type;
+
+  template <typename T>
+  struct force_optional
+  {
+    using type = ::sqlpp::optional<T>;
+  };
+
+  template <typename T>
+  struct force_optional<::sqlpp::optional<T>>
+  {
+    using type = ::sqlpp::optional<T>;
+  };
+
+  template <typename T>
+  using force_optional_t = typename force_optional<T>::type;
+
+
+}
diff --git a/include/sqlpp11/core/type_traits/value_type.h b/include/sqlpp11/core/type_traits/value_type.h
new file mode 100644
index 00000000..a556ffd7
--- /dev/null
+++ b/include/sqlpp11/core/type_traits/value_type.h
@@ -0,0 +1,237 @@
+#pragma once
+
+/*
+ * Copyright (c) 2024, 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 <cstdint>
+
+#include <sqlpp11/core/chrono.h>
+#include <sqlpp11/core/compat/optional.h>
+#include <sqlpp11/core/type_traits/optional.h>
+
+namespace sqlpp
+{
+  struct no_value_t;
+
+  template <typename T>
+  struct value_type_of
+  {
+    using type = no_value_t;
+  };
+
+  template <typename T>
+  using value_type_of_t = typename value_type_of<T>::type;
+
+  template<typename T>
+  struct value_type_of<::sqlpp::optional<T>>
+  {
+    using type = sqlpp::optional<value_type_of_t<remove_optional_t<T>>>;
+  };
+
+  template <typename T>
+  struct has_value_type : public std::integral_constant<bool, not std::is_same<value_type_of_t<T>, no_value_t>::value> {};
+
+  struct boolean{};
+  template<>
+  struct value_type_of<boolean> { using type = boolean; };
+  template<>
+  struct value_type_of<bool> { using type = boolean; };
+
+  struct integral{};
+  template<>
+  struct value_type_of<integral> { using type = integral; };
+  template<>
+  struct value_type_of<int8_t> { using type = integral; };
+  template<>
+  struct value_type_of<int16_t> { using type = integral; };
+  template<>
+  struct value_type_of<int32_t> { using type = integral; };
+  template<>
+  struct value_type_of<int64_t> { using type = integral; };
+
+  struct unsigned_integral{};
+  template<>
+  struct value_type_of<unsigned_integral> { using type = unsigned_integral; };
+  template<>
+  struct value_type_of<uint8_t> { using type = unsigned_integral; };
+  template<>
+  struct value_type_of<uint16_t> { using type = unsigned_integral; };
+  template<>
+  struct value_type_of<uint32_t> { using type = unsigned_integral; };
+  template<>
+  struct value_type_of<uint64_t> { using type = unsigned_integral; };
+
+  struct floating_point{};
+  template<>
+  struct value_type_of<floating_point> { using type = floating_point; };
+  template<>
+  struct value_type_of<float> { using type = floating_point; };
+  template<>
+  struct value_type_of<double> { using type = floating_point; };
+  template<>
+  struct value_type_of<long double> { using type = floating_point; };
+
+  struct text{};
+  template <>
+  struct value_type_of<text> { using type = text; };
+  template <>
+  struct value_type_of<char> { using type = text; };
+  template <>
+  struct value_type_of<const char*> { using type = text; };
+  template <>
+  struct value_type_of<std::string> { using type = text; };
+  template <>
+  struct value_type_of<::sqlpp::string_view> { using type = text; };
+
+  struct blob{};
+  template <>
+  struct value_type_of<blob> { using type = blob; };
+  template <std::size_t N>
+  struct value_type_of<std::array<std::uint8_t, N>> { using type = blob; };
+  template <>
+  struct value_type_of<std::vector<std::uint8_t>> { using type = blob; };
+  template <>
+  struct value_type_of<::sqlpp::span<std::uint8_t>> { using type = blob; };
+
+  struct day_point{};
+  template <>
+  struct value_type_of<day_point> { using type = day_point; };
+  template <>
+  struct value_type_of<std::chrono::time_point<std::chrono::system_clock, sqlpp::chrono::days>> { using type = day_point; };
+
+  struct time_of_day{};
+  template <>
+  struct value_type_of<time_of_day> { using type = time_of_day; };
+  template <typename Rep, typename Period>
+  struct value_type_of<std::chrono::duration<Rep, Period>> { using type = time_of_day; };
+
+  struct time_point{};
+  template <>
+  struct value_type_of<time_point> { using type = time_point; };
+  template <typename Period>
+  struct value_type_of<std::chrono::time_point<std::chrono::system_clock, Period>> { using type = time_point; };
+
+
+  template <typename T>
+  struct is_boolean : public std::is_same<remove_optional_t<value_type_of_t<T>>, boolean>
+  {
+  };
+
+  template <>
+  struct is_boolean<::sqlpp::nullopt_t> : public std::true_type {
+  };
+
+  template <typename T>
+  struct is_integral : public std::is_same<remove_optional_t<value_type_of_t<T>>, integral>
+  {
+  };
+
+  template <>
+  struct is_integral<::sqlpp::nullopt_t> : public std::true_type {
+  };
+
+  template <typename T>
+  struct is_unsigned_integral : public std::is_same<remove_optional_t<value_type_of_t<T>>, unsigned_integral>
+  {
+  };
+
+  template <>
+  struct is_unsigned_integral<::sqlpp::nullopt_t> : public std::true_type {
+  };
+
+  template <typename T>
+  struct is_floating_point : public std::is_same<remove_optional_t<value_type_of_t<T>>, floating_point>
+  {
+  };
+
+  template <>
+  struct is_floating_point<::sqlpp::nullopt_t> : public std::true_type {
+  };
+
+  // A generic numeric type which could be (unsigned) integral or floating point.
+  struct numeric{};
+  template <typename T>
+  struct is_numeric
+      : public std::integral_constant<bool,
+                                      is_boolean<T>::value or is_integral<T>::value or is_unsigned_integral<T>::value or
+                                          is_floating_point<T>::value or
+                                          std::is_same<remove_optional_t<value_type_of_t<T>>, numeric>::value>
+  {
+  };
+
+  template <>
+  struct is_numeric<::sqlpp::nullopt_t> : public std::true_type{};
+
+  template <typename T>
+  struct is_text : public std::is_same<remove_optional_t<value_type_of_t<T>>, text>
+  {
+  };
+
+  template <>
+  struct is_text<::sqlpp::nullopt_t> : public std::true_type {
+  };
+
+  template <typename T>
+  struct is_blob : public std::is_same<remove_optional_t<value_type_of_t<T>>, blob>
+  {
+  };
+
+  template <>
+  struct is_blob<::sqlpp::nullopt_t> : public std::true_type {
+  };
+
+  template <typename T>
+  struct is_day_point : public std::is_same<remove_optional_t<value_type_of_t<T>>, day_point>
+  {
+  };
+
+  template <>
+  struct is_day_point<::sqlpp::nullopt_t> : public std::true_type {
+  };
+
+  template <typename T>
+  struct is_time_point : public std::is_same<remove_optional_t<value_type_of_t<T>>, time_point>
+  {
+  };
+
+  template <>
+  struct is_time_point<::sqlpp::nullopt_t> : public std::true_type {
+  };
+
+  template <typename T>
+  struct is_day_or_time_point : public std::integral_constant<bool, is_day_point<T>::value or is_time_point<T>::value> {
+  };
+
+  template <typename T>
+  struct is_time_of_day : public std::is_same<remove_optional_t<value_type_of_t<T>>, time_of_day>
+  {
+  };
+
+  template <>
+  struct is_time_of_day<::sqlpp::nullopt_t> : public std::true_type {
+  };
+
+}
diff --git a/tests/core/types/CMakeLists.txt b/tests/core/types/CMakeLists.txt
index 37771630..442511eb 100644
--- a/tests/core/types/CMakeLists.txt
+++ b/tests/core/types/CMakeLists.txt
@@ -37,4 +37,5 @@ test_compile(value)
 
 add_subdirectory(aggregate_function)
 add_subdirectory(operator)
+add_subdirectory(clause)
 add_subdirectory(type_traits)
diff --git a/tests/core/types/clause/CMakeLists.txt b/tests/core/types/clause/CMakeLists.txt
new file mode 100644
index 00000000..a895c99c
--- /dev/null
+++ b/tests/core/types/clause/CMakeLists.txt
@@ -0,0 +1,33 @@
+# Copyright (c) 2024, 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.
+
+function(test_compile name)
+    set(target sqlpp11_core_types_clause_${name})
+    add_executable(${target} ${name}.cpp)
+    target_link_libraries(${target} PRIVATE sqlpp11::sqlpp11 sqlpp11_testing)
+endfunction()
+
+test_compile(group_by)
+test_compile(select_columns)
+
diff --git a/tests/core/types/clause/group_by.cpp b/tests/core/types/clause/group_by.cpp
new file mode 100644
index 00000000..4d8ed5e1
--- /dev/null
+++ b/tests/core/types/clause/group_by.cpp
@@ -0,0 +1,68 @@
+/*
+ * Copyright (c) 2024, 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 "Sample.h"
+#include <sqlpp11/sqlpp11.h>
+
+namespace
+{
+  template <typename... T>
+  auto known_aggregate_columns_as_expected(T... t)
+      -> std::is_same<sqlpp::known_aggregate_columns_of_t<decltype(group_by(std::move(t)...))>,
+                      sqlpp::detail::type_vector<T...>>;
+}
+
+void test_group_by()
+{
+  auto v = sqlpp::value(17);
+  auto col_int = test::TabFoo{}.id;
+  auto col_txt = test::TabFoo{}.textNnD;
+
+  // Constant values are neutral and therefore considered neither aggregate and non-aggregate.
+  static_assert(decltype(known_aggregate_columns_as_expected(col_int))::value, "");
+  static_assert(decltype(known_aggregate_columns_as_expected(col_int, col_txt))::value, "");
+
+  // Dynamic columns are listed as dynamic columns in known_aggregate_columns_of_t, since they need to be dynamic in
+  // select, too, and we need to check that there.
+  static_assert(decltype(known_aggregate_columns_as_expected(dynamic(true, col_int)))::value, "");
+  static_assert(decltype(known_aggregate_columns_as_expected(col_int, dynamic(true, col_txt)))::value, "");
+
+  // Declared columns are listed as declared columns in known_aggregate_columns_of_t, since they need to be declared in
+  // select, too, and we need to check that there.
+  static_assert(decltype(known_aggregate_columns_as_expected(declare_group_by_column(v)))::value, "");
+  static_assert(decltype(known_aggregate_columns_as_expected(col_int, declare_group_by_column(v)))::value, "");
+
+  // Declared columns can be dynamic, too.
+  static_assert(decltype(known_aggregate_columns_as_expected(dynamic(true, declare_group_by_column(v))))::value, "");
+  static_assert(decltype(known_aggregate_columns_as_expected(col_int, dynamic(true, declare_group_by_column(v))))::value, "");
+
+#warning: need to test dynamic and declared columns in select, too
+}
+
+int main()
+{
+  void test_group_by();
+}
+
diff --git a/tests/core/types/clause/select_columns.cpp b/tests/core/types/clause/select_columns.cpp
new file mode 100644
index 00000000..d7b744ab
--- /dev/null
+++ b/tests/core/types/clause/select_columns.cpp
@@ -0,0 +1,53 @@
+/*
+ * Copyright (c) 2024, 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 "Sample.h"
+#include <sqlpp11/sqlpp11.h>
+
+namespace
+{
+  SQLPP_ALIAS_PROVIDER(cheese);
+}
+
+void test_select_columns()
+{
+  auto v = sqlpp::value(17);
+  auto col_int = test::TabFoo{}.id;
+  auto col_txt = test::TabFoo{}.textNnD;
+
+  select_columns(col_int);
+  select_columns(dynamic(true, col_int));
+  select_columns(as(declare_group_by_column(v), cheese));
+  select_columns(dynamic(true, declare_group_by_column(v)).as(cheese));
+
+#warning: add actual tests here
+
+}
+
+int main()
+{
+  void test_group_by();
+}
+
diff --git a/tests/core/types/type_traits/CMakeLists.txt b/tests/core/types/type_traits/CMakeLists.txt
index 2786010e..c40cd99d 100644
--- a/tests/core/types/type_traits/CMakeLists.txt
+++ b/tests/core/types/type_traits/CMakeLists.txt
@@ -23,7 +23,7 @@
 # SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 function(test_compile name)
-    set(target sqlpp11_core_types_operator_${name})
+    set(target sqlpp11_core_types_type_traits_${name})
     add_executable(${target} ${name}.cpp)
     target_link_libraries(${target} PRIVATE sqlpp11::sqlpp11 sqlpp11_testing)
 endfunction()