Introduce optional select columns

Basic functionality seems to work, more tests needed.

include/sqlpp11/column.h

@@ -86,6 +86,11 @@ namespace sqlpp
       return {*this};
     }
 
+    sqlpp::compat::optional<column_t> if_(bool condition) const
+    {
+      return condition ? sqlpp::compat::make_optional(*this) : sqlpp::compat::nullopt;
+    }
+
     template <typename T>
     auto operator=(T t) const -> assignment_t<column_t, wrap_operand_t<T>>
     {

include/sqlpp11/compat/optional.h

@@ -45,6 +45,8 @@ namespace sqlpp
   using std::nullopt;
 
   using std::bad_optional_access;
+  using std::make_optional;
+
   }  // namespace compat
 }  // namespace sqlpp
 
@@ -249,6 +251,12 @@ namespace sqlpp
       return !right;
     }
 
+    template <typename T>
+    constexpr optional<typename std::decay<T>::type> make_optional(T&& value)
+    {
+      return optional<typename std::decay<T>::type>(std::forward<T>(value));
+    }
+
   }  // namespace compat
 }  // namespace sqlpp
 

include/sqlpp11/field_spec.h

@@ -76,13 +76,14 @@ namespace sqlpp
     template <typename Select, typename NamedExpr>
     struct make_field_spec_impl
     {
-      static constexpr bool _can_be_null = can_be_null_t<NamedExpr>::value;
+      using RawNamedExpr = remove_optional_t<NamedExpr>;
+      static constexpr bool _can_be_null = is_optional<NamedExpr>::value or can_be_null_t<RawNamedExpr>::value;
       static constexpr bool _depends_on_outer_table =
-          detail::make_intersect_set_t<required_tables_of<NamedExpr>,
+          detail::make_intersect_set_t<required_tables_of<RawNamedExpr>,
                                        typename Select::_used_outer_tables>::size::value > 0;
 
-      using type = field_spec_t<typename NamedExpr::_alias_t,
-                                value_type_of<NamedExpr>,
+      using type = field_spec_t<typename RawNamedExpr::_alias_t,
+                                value_type_of<RawNamedExpr>,
                                 logic::any_t<_can_be_null, _depends_on_outer_table>::value>;
     };
   }  // namespace detail

include/sqlpp11/select_column_list.h

@@ -55,36 +55,44 @@ namespace sqlpp
     template <typename Column>
     struct select_traits<Column>
     {
-      using _traits = make_traits<value_type_of<Column>,
+      using _traits = make_traits<value_type_of<remove_optional_t<Column>>,
                                   tag::is_select_column_list,
                                   tag::is_return_value,
                                   tag::is_expression,
                                   tag::is_selectable>;
-      using _alias_t = typename Column::_alias_t;
+      using _alias_t = typename remove_optional_t<Column>::_alias_t;
     };
   }  // namespace detail
 
-  // SELECTED COLUMNS DATA
-  template <typename... Columns>
-  struct select_column_list_data_t
+  template <typename Column>
+  struct select_column_t
   {
-    select_column_list_data_t(Columns... columns) : _columns(columns...)
+    constexpr select_column_t(Column column) : _column(column)
     {
     }
 
-    select_column_list_data_t(std::tuple<Columns...> columns) : _columns(columns)
-    {
-    }
+    constexpr select_column_t(const select_column_t&) = default;
+    constexpr select_column_t(select_column_t&&) = default;
+    select_column_t& operator=(const select_column_t&) = default;
+    select_column_t& operator=(select_column_t&&) = default;
+    ~select_column_t() = default;
 
-    select_column_list_data_t(const select_column_list_data_t&) = default;
-    select_column_list_data_t(select_column_list_data_t&&) = default;
-    select_column_list_data_t& operator=(const select_column_list_data_t&) = default;
-    select_column_list_data_t& operator=(select_column_list_data_t&&) = default;
-    ~select_column_list_data_t() = default;
-
-    std::tuple<Columns...> _columns;
+    Column _column;
   };
 
+  template <typename Context, typename Column>
+  Context& serialize(const select_column_t<Column>& t, Context& context)
+  {
+    if (has_value(t._column))
+    {
+      return serialize(get_value(t._column), context);
+    }
+
+    context << "NULL AS " << name_of<remove_optional_t<Column>>::template char_ptr<Context>();
+    return context;
+  }
+
+
   SQLPP_PORTABLE_STATIC_ASSERT(
       assert_no_unknown_tables_in_selected_columns_t,
       "at least one selected column requires a table which is otherwise not known in the statement");
@@ -103,7 +111,7 @@ namespace sqlpp
 
     using _alias_t = typename detail::select_traits<Columns...>::_alias_t;
 
-    using _data_t = select_column_list_data_t<Columns...>;
+    using _data_t = std::tuple<select_column_t<Columns>...>;
 
     struct _column_type
     {
@@ -231,8 +239,9 @@ namespace sqlpp
   template <typename... T>
   struct check_selected_columns
   {
-    using type = static_combined_check_t<
-        static_check_t<logic::all_t<is_selectable_t<T>::value...>::value, assert_selected_colums_are_selectable_t>>;
+    using type =
+        static_combined_check_t<static_check_t<logic::all_t<is_selectable_t<remove_optional_t<T>>::value...>::value,
+                                               assert_selected_colums_are_selectable_t>>;
   };
   template <typename... T>
   using check_selected_columns_t = typename check_selected_columns<T...>::type;
@@ -299,9 +308,9 @@ namespace sqlpp
 
   // Interpreters
   template <typename Context, typename... Columns>
-  Context& serialize(const select_column_list_data_t<Columns...>& t, Context& context)
+  Context& serialize(const std::tuple<select_column_t<Columns>...>& t, Context& context)
   {
-    interpret_tuple(t._columns, ',', context);
+    interpret_tuple(t, ',', context);
     return context;
   }
 

include/sqlpp11/type_traits.h

@@ -28,6 +28,8 @@
 
 #include <type_traits>
 #include <tuple>
+
+#include <sqlpp11/compat/optional.h>
 #include <sqlpp11/consistent.h>
 #include <sqlpp11/portable_static_assert.h>
 #include <sqlpp11/detail/type_vector.h>
@@ -37,6 +39,55 @@
 
 namespace sqlpp
 {
+  template <typename T>
+  struct is_optional : public std::false_type
+  {
+  };
+
+  template <typename T>
+  struct is_optional<sqlpp::compat::optional<T>> : public std::true_type
+  {
+  };
+
+  template <typename T>
+  struct remove_optional
+  {
+    using type = T;
+  };
+
+  template <typename T>
+  struct remove_optional<sqlpp::compat::optional<T>>
+  {
+    using type = T;
+  };
+
+  template <typename T>
+  const T& get_value(const T& t)
+  {
+    return t;
+  }
+
+  template <typename T>
+  const T& get_value(const sqlpp::compat::optional<T>& t)
+  {
+    return t.value();
+  }
+
+  template <typename T>
+  auto has_value(const T&) -> bool
+  {
+    return true;
+  }
+
+  template <typename T>
+  auto has_value(const sqlpp::compat::optional<T>& t) -> bool
+  {
+    return t.has_value();
+  }
+
+  template <typename T>
+  using remove_optional_t = typename remove_optional<T>::type;
+
   struct no_value_t;
   namespace detail
   {

tests/core/serialize/SelectColumns.cpp

@@ -39,21 +39,37 @@ int SelectColumns(int, char*[])
   compare(__LINE__, select(foo.doubleN, bar.id), "SELECT tab_foo.double_n,tab_bar.id");
 
   // All columns of a table
-  compare(__LINE__, select(all_of(foo)), "SELECT tab_foo.id,tab_foo.text_nn_d,tab_foo.int_n,tab_foo.double_n,tab_foo.u_int_n,tab_foo.blob_n");
+  compare(__LINE__, select(all_of(foo)),
+          "SELECT tab_foo.id,tab_foo.text_nn_d,tab_foo.int_n,tab_foo.double_n,tab_foo.u_int_n,tab_foo.blob_n");
 
   // All columns of a table plus one more
-  compare(__LINE__, select(all_of(foo), bar.id), "SELECT tab_foo.id,tab_foo.text_nn_d,tab_foo.int_n,tab_foo.double_n,tab_foo.u_int_n,tab_foo.blob_n,tab_bar.id");
+  compare(
+      __LINE__, select(all_of(foo), bar.id),
+      "SELECT tab_foo.id,tab_foo.text_nn_d,tab_foo.int_n,tab_foo.double_n,tab_foo.u_int_n,tab_foo.blob_n,tab_bar.id");
 
   // One more, plus all columns of a table
-  compare(__LINE__, select(bar.id, all_of(foo)), "SELECT tab_bar.id,tab_foo.id,tab_foo.text_nn_d,tab_foo.int_n,tab_foo.double_n,tab_foo.u_int_n,tab_foo.blob_n");
+  compare(
+      __LINE__, select(bar.id, all_of(foo)),
+      "SELECT tab_bar.id,tab_foo.id,tab_foo.text_nn_d,tab_foo.int_n,tab_foo.double_n,tab_foo.u_int_n,tab_foo.blob_n");
 
   // Column and aggregate function
   compare(__LINE__, select(foo.doubleN, count(bar.id)), "SELECT tab_foo.double_n,COUNT(tab_bar.id) AS count_");
 
   // Column aliases
-  compare(__LINE__, select(foo.doubleN.as(sqlpp::alias::o), count(bar.id).as(sqlpp::alias::a)), "SELECT tab_foo.double_n AS o,COUNT(tab_bar.id) AS a");
+  compare(__LINE__, select(foo.doubleN.as(sqlpp::alias::o), count(bar.id).as(sqlpp::alias::a)),
+          "SELECT tab_foo.double_n AS o,COUNT(tab_bar.id) AS a");
 
-#warning: add optional column tests
+  // Optional column manually
+  compare(__LINE__, select(sqlpp::compat::make_optional(bar.id)), "SELECT tab_bar.id");
+  auto opt_id = sqlpp::compat::make_optional(bar.id);
+  opt_id.reset();
+  compare(__LINE__, select(opt_id), "SELECT NULL AS id");
+
+  // Optional column using `if_`
+  compare(__LINE__, select(bar.id.if_(true)), "SELECT tab_bar.id");
+  compare(__LINE__, select(bar.id.if_(false)), "SELECT NULL AS id");
+
+#warning: add more optional column tests
 
   return 0;
 }

tests/core/types/result_row.cpp

@@ -55,6 +55,21 @@ namespace
     }
   }
 
+  void optional_columns()
+  {
+    {
+      // result fields are as nullable as the expressions they represent
+#warning: add `if_` to other expressions, too
+#warning: test with nullable columns, too.
+#warning: test with all kinds of functions as well.
+#warning: We should actually test for the exact type!
+      const auto rows = db(select(bar.id.if_(true), bar.boolNn.if_(true)/*, seven*/).from(bar).unconditionally());
+      auto& x = rows.front();
+      static_assert(is_optional<decltype(x.id)>::value, "");
+      static_assert(is_optional<decltype(x.boolNn)>::value, "");
+    }
+  }
+
   void join()
   {
     // Join
@@ -233,6 +248,7 @@ namespace
 int main(int, char* [])
 {
   single_table();
+  optional_columns();
   join();
   aggregates();
 }