MakeSelecter() [2/2]

template<EntityValueType ENTITY>

template<ColumnType... COLUMNS>

constexpr auto sqt::DataContext< ENTITY >::MakeSelecter ( const COLUMNS &... columns )

staticconstexprnoexcept

Creates a selecter for retrieving the specified columns from the database table.

Template Parameters

COLUMNS

A pack of the column types.

Parameters

columns

The columns to be retrieved.

Returns

A new selecter instance, whose result element type is a composite value type (a std::tuple<> of the specified columns' value types).

This method is similar to the MakeSelecter() method, with the following differences:

• The returned selecter selects only the specified columns of the table, rather than all columns.
• The result element type is a composite value type rather than the entity type.

The following code demonstrates how to use the selecter:

// The entity type, assuming its table type has been defined and registered, and its
// columns' names are defined as the same as the entity's field names.
struct MyEntity {
    int id{};
    std::string name;
};
 
// Create the selecter with two columns.
auto selecter = sqt::DataContext<MyEntity>::MakeSelecter(
    sqt::Table<MyEntity>.id,
    sqt::Table<MyEntity>.name,
);
 
// Create a data context, assuming the shared_db is an opened database instance.
sqt::DataContext<MyEntity> data_context{ shared_db };
 
// Prepare the selecter to create a corresponding executor.
auto executor = data_context.Prepare(selecter);
 
// Execute the statement to obtain the result.
auto result = executor.Execute();
 
// Iterate through the result and print the tuples.
for (const std::tuple<int, std::string>& tuple : result) {
    std::cout << "ID: " << std::get<0>(tuple) << ", Name: " << std::get<1>(tuple) << '\n';
}

See also

sqt::DataContext<>::MakeSelecter()

sqt::DataContext<>::Select()

sqt::DataContext<>::SelectAll()