EXISTS

The EXISTS keyword tests whether rows exist in the result of a subquery. True is returned if the subquery contains any rows.

Syntax

EXISTS ( subquery )

Remarks

For every row the outer query evaluates, PSQL tests for the existence of a related row from the subquery. PSQL includes in the statement's result table each row from the outer query that corresponds to a related row from the subquery.

You may use EXISTS for a subquery within a stored procedure. However, the subquery SELECT statement within the stored procedure may not contain a COMPUTE clause or the INTO keyword.

In most cases, a subquery with EXISTS can be rewritten to use IN. PSQL can process the query more efficiently if the query uses IN.

Examples

The following statement returns a list containing only persons who have a 4.0 grade point average:

SELECT * FROM Person p WHERE EXISTS

(SELECT * FROM Enrolls e WHERE e.Student_ID = p.id

AND Grade = 4.0)

This statement can be rewritten to use IN:

SELECT * FROM Person p WHERE p.id IN

(SELECT e.Student_ID FROM Enrolls WHERE Grade = 4.0)

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The following procedure selects the ID from the Person table using a value as an input parameter. The first EXEC of the procedure returns “Exists returned true.” The second EXEC returns “Exists returned false.”

CREATE PROCEDURE ex1(IN :vID INTEGER) AS

BEGIN

IF EXISTS ('SELECT id FROM person WHERE id < :vID)

THEN PRINT 'Exists returned true';

ELSE PRINT 'Exists returned false';

END IF;

END;

EXEC ex1(222222222)

EXEC ex1(1)