Checks whether a subquery produces any rows of query results
[ NOT ] EXISTS ( subquery )
The EXISTS condition is TRUE if the subquery result contains at least one row, and FALSE if the subquery result does not contain any rows. The EXISTS condition cannot be UNKNOWN.
You can reverse the logic of the EXISTS condition by using the NOT EXISTS form. In this case, the test returns TRUE if the subquery produces no rows, and FALSE otherwise.
List the customers who placed orders after July 13, 2001.
SELECT GivenName, Surname FROM Customers WHERE EXISTS ( SELECT * FROM SalesOrders WHERE (OrderDate > '2001-07-13') AND (Customers.ID = SalesOrders.CustomerID))