CREATE OR REPLACE FUNCTION funkcjaRabat
(p_id customers.customer%TYPE) RETURN NUMBER
IS
v_rabat book.retail%TYPE;
v_ilosc book.quantity%TYPE;
BEGIN

SELECT oi.quantity INTO v_ilosc,
CASE
WHEN v_ilosc > 10 than  3
WHEN v_ilosc < 10 AND  v_ilosc > 4 than  0
END v_rabat
FROM orderitems oi, orders o
WHERE  p_id = o.customer AND o.ORDER = oi.ORDER;

RETURN (v_rabat);
END funkcjaRabat;


CREATE OR REPLACE TRIGGER triggerRabat
BEFORE INSERT ON Books FOR EACH ROW
BEGIN
   :NEW.retail:=funkcjaRabat.NEXTVAL;
END;
/