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, Zen tests for the existence of a related row from the subquery. Zen 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. Zen 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)
============
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)