A subquery is a SELECT statement used as part of the selection criteria of the main SELECT statement. The subquery specified in the WHERE clause will be evaluated repeated on each row of the selection base table. The output of the subquery will be used in the final evaluation of the criteria. Usually, subqueries are used in the following Boolean operations:

* "expression IN (subquery)"

* "expression NOT IN (subquery)"

* "EXISTS (subquery)"

* "NOT EXISTS (subquery)"