Suppose you want to prevent undeliverable email from being stored in a table named ELECTRONIC_MAIL. To do so, you must prevent the execution of certain SQL INSERT statements.
CREATE TRIGGER BLOCK_INSERT
NO CASCADE BEFORE INSERT ON ELECTRONIC_MAIL
REFERENCING NEW AS N
FOR EACH ROW
WHEN (SUBJECT(N.MESSAGE) = 'undelivered mail')
BEGIN ATOMIC
SIGNAL SQLSTATE '85101'
SET MESSAGE_TEXT = ('Attempt to insert undelivered mail');
END
ALTER TABLE ELECTRONIC_MAIL
ADD CONSTRAINT NO_UNDELIVERED
CHECK (SUBJECT <> 'undelivered mail')