Classe « Query »

Méthode sqlalchemy.orm.Query.exists

Signature de la méthode exists

def exists(self) 

Description

exists.__doc__

A convenience method that turns a query into an EXISTS subquery
        of the form EXISTS (SELECT 1 FROM ... WHERE ...).

        e.g.::

            q = session.query(User).filter(User.name == 'fred')
            session.query(q.exists())

        Producing SQL similar to::

            SELECT EXISTS (
                SELECT 1 FROM users WHERE users.name = :name_1
            ) AS anon_1

        The EXISTS construct is usually used in the WHERE clause::

            session.query(User.id).filter(q.exists()).scalar()

        Note that some databases such as SQL Server don't allow an
        EXISTS expression to be present in the columns clause of a
        SELECT.    To select a simple boolean value based on the exists
        as a WHERE, use :func:`.literal`::

            from sqlalchemy import literal

            session.query(literal(True)).filter(q.exists()).scalar()