The EXISTS predicate (exists_predicate) checks whether a result table (see result table name) contains at least one row.
<exists_predicate> ::= EXISTS <subquery>
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.
SELECT cno, title, name, zip FROM customer WHERE
EXISTS
(SELECT * FROM reservation WHERE customer.cno =
reservation.cno)
Only select customers that have one or more reservations.
CNO |
TITLE |
NAME |
ZIP |
3000 |
Mrs |
Porter |
10580 |
3100 |
Mr |
Brown |
48226 |
3200 |
Company |
Datasoft |
90018 |
3600 |
Mrs |
Smith |
75243 |
3900 |
Mr |
Howe |
75243 |
4100 |
Mrs |
Baker |
90018 |
4300 |
Company |
TOOLware |
20019 |
4400 |
Mr |
Jenkins |
20903 |