1. Triggers Examples

2. 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.

3. (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.

4. (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.

5. 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'