The following WHEN clause is valid because an aggregate appears on the right-hand side of the search condition, and the left-hand side of the inequality condition is a constant.
CREATE TRIGGER TrigWhen AFTER INSERT ON t1 REFERENCING NEW AS NewRow FOR EACH ROW WHEN (10 > ( SELECT SUM(b) FROM t2 WHERE t2.c < 5)) ABORT;
If you insert values into t1 and the WHEN condition is satisfied, then the triggered action statement, ABORT, performs and a failure message is returned.
INSERT INTO t1 (1, 1, 1);
Result:
*** Failure 3514 User-generated transaction ABORT.