MakeSelecter() [1/2]

template<EntityValueType ENTITY>

constexpr auto sqt::DataContext< ENTITY >::MakeSelecter ( )

staticconstexprnoexcept

Creates a selecter for retrieving entities from the database table.

Returns

A new selecter instance, whose result element type is the entity type.

The returned selecter corresponds to a SELECT SQL statement without any conditions, so it will select all entities from the table. To apply select conditions, use the Where(), OrderBy(), and Limit() methods of the selecter to create a new selecter with conditions. 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 a condition.
auto selecter = sqt::DataContext<MyEntity>::MakeSelecter().Where(
    sqt::Table<MyEntity>.name != "Unknown"
);
 
// 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 entities.
for (const MyEntity& entity : result) {
    std::cout << "ID: " << entity.id << ", Name: " << entity.name << '\n';
}

The returned selecter selects all columns of the table. To control which columns to select, use the MakeSelecter(const COLUMN&&...) method instead.

For easier-to-use methods, use the following methods from the easy style interface:

• Select() for selecting a single entity by the specified primary key value.
• SelectAll() for selecting all entities in the table.

See also

sqt::DataContext<>::MakeSelecter(const COLUMNS&... columns);

sqt::DataContext<>::Select()

sqt::DataContext<>::SelectAll()