此新租借历史记录表的触发器,可防止从表中删除.
CREATE OR REPLACE TRIGGER RENTALHIS_DEL
BEFORE DELETE ON RENTALHISTORY
BEGIN
dbms_output.put_line( 'Records can not be deleted');
END;
DELETE FROM RENTALHISTORY WHERE RENTALID = 1;
-- It is deleting before it says it can not delete
1 rows deleted.
Records can not be deleted