sqt::QuerierType Concept Reference

#include <sqt/orm/querier/querier_type.h>

Description

Constrains a type to be a querier type, which corresponds to a SQL statement that can be executed.

Requirements

• The type must have a static constant ParameterIndex of type std::size_t, with a value greater than 0.
• The type must have a static constant ParameterCount of type std::size_t.
• The type must have a static method BuildSQL() with the following signature:

  static std::string_view BuildSQL();

• The type must have a static method BuildPlaceholderBinders() with the following signature:

  static auto BuildPlaceholderBinders() noexcept;

  The return type must satisfy the sqt::BinderTupleType concept.
• The type must have a member function BindInlineParameters() with the following signature:

  void BindInlineParameters(sqt::Statement& statement) const;

A querier is an encapsulation for a SQL statement. Each querier instance corresponds to a well-formed SQL statement that can be executed. There are four types of queriers:

• Inserter, which corresponds to an INSERT SQL statement.
• Updater, which corresponds to an UPDATE SQL statement.
• Deleter, which corresponds to a DELETE SQL statement.
• Selecter, which corresponds to a SELECT SQL statement, constrained by the sqt::SelecterType concept.

The Make*() methods in sqt::DataContext<> are used to create queriers, referred to as primary queriers, which contain the core components of a SQL statement. For instance:

// 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 a primary selecter that selects all entities.
constexpr auto selecter = sqt::DataContext<MyEntity>::MakeSelecter();

The above code creates a primary selecter corresponds to the following SQL statement:

SELECT id, name FROM MyEntity;

Querier decorators add additional clauses to the SQL statement. They are created by decorative methods of queriers (either primary queriers or querier decorators). For instance:

constexpr auto selecter = sqt::DataContext<MyEntity>::MakeSelecter()
    .Where(sqt::Table<MyEntity>.name != "Unknown")
    .OrderBy(sqt::Table<MyEntity>.id.Desc());

The above code creates a selecter decorator in chain fashion. The decorator corresponds to the following SQL statement:

SELECT id, name FROM MyEntity WHERE name <> "Unknown" ORDER BY id DESC;

See also

sqt::BinderTupleType

sqt::DataContext<>

sqt::SelecterType