The following CREATE TRIGGER request succeeds because its WHEN clause aggregate function is specified as part of a subquery:
Note that the aggregate subquery is on the right hand side of the inequality condition, as it must be.
CREATE TRIGGER trigwhen3 BEFORE INSERT ON t1 REFERENCING OLD AS previous FOR EACH ROW WHEN (previous.a <= ( SELECT SUM(b) FROM t2, t3 WHERE t2.c = t3.c)) ABORT;