EXISTS predicate 

The EXISTS predicate checks whether a result table (see result table name) contains at least one row.

Syntax

<exists_predicate> ::= EXISTS <subquery>

subquery

Explanation

The truth content of an EXISTS predicate is either true or false.

The subquery generates a result table. If this result table contains at least one row, EXISTS <subquery> is true.

Model table customer, reservation

Only select customers that have one or more reservations:

SELECT * FROM customer WHERE EXISTS
(SELECT * FROM reservation WHERE customer.cno = reservation.cno)