Triggers Examples

Exercise – PCs, Laptops, Printers Product(maker, model, type) PC(model, speed, ram, hd, rd, price) Laptop(model, speed, ram, hd, screen, price) Printer(model, color, type, price) Create triggers to enforce the following business rules. A model of a product must also be the model of a PC, a laptop, or a printer. When updating the price of a PC, check that there is no lower priced PC with the same speed.

(a) CREATE OR REPLACE TRIGGER ProductModelCheck AFTER INSERT on Product FOR EACH ROW DECLARE --local variables model_local INT; CURSOR model_check IS SELECT model FROM ( (SELECT model from PC) UNION (SELECT model from Laptop) UNION (SELECT model from Printer) ) WHERE model=:new.model; BEGIN OPEN model_check; FETCH model_check INTO model_local; IF model_check%NOTFOUND THEN RAISE_APPLICATION_ERROR(-20000, 'No model number ' || :new.model || ' in PC, Laptop or Printer'); END IF; END ProductModelCheck; Now try: INSERT INTO Product (model, maker, type) VALUES (9999, 'Alex', 'PC'); Since model 9999 isn’t yet in PC, the above will fail.

(b) CREATE OR REPLACE TRIGGER CHECK_PRICE BEFORE UPDATE OF price ON PC FOR EACH ROW DECLARE pragma autonomous_transaction; price_min PC.price%TYPE; CURSOR pc_cur IS SELECT MIN(price) FROM PC WHERE speed=:new.speed; BEGIN OPEN pc_cur; FETCH pc_cur INTO price_min; IF price_min IS NOT NULL AND price_min < :old.price THEN RAISE_APPLICATION_ERROR(-20000, 'price rule violated. Lower price is: ' || price_min); END IF; CLOSE pc_cur; END; Needed in ORACLE if you are accessing the same table that caused the trigger to fire.

Now try… INSERT INTO PC(model,speed,ram,hd,rd,price) VALUES(1111,3,4,80,24,1500); INSERT INTO Product(model,maker,type) VALUES(1111,'DELL','PC'); COMMIT; VALUES(2222,3,4,120,32,1000); VALUES(2222,'IBM','PC'); UPDATE PC SET price=1400 WHERE model=1111; This update shouldn’t go through according to our constraint, and indeed, it gives: Error report: SQL Error: ORA-20000: price rule violated. Lower price is: 1000 ORA-06512: at "THOMO.CHECK_PRICE", line 15 ORA-04088: error during execution of trigger 'THOMO.CHECK_PRICE'