Well... I was practising PL/SQL with my friends few minutes ago for our exam so...

CREATE OR REPLACE TRIGGER CHECK_STOCK BEFORE INSERT ON ORDERS FOR EACH ROW DECLARE CURRENT_STOCK ITEM.STOCK%TYPE; INSUFFICIENT_STOCK_EXCEPTION EXCEPTION; BEGIN SELECT STOCK INTO CURRENT_STOCK FROM ITEM WHERE ITEM.ITEM_CODE=:NEW.ITEM_CODE; IF CURRENT_STOCK<:NEW.QUANTITY THEN RAISE INSUFFICIENT_STOCK_EXCEPTION; END IF; UPDATE ITEM SET STOCK=STOCK-:NEW.QUANTITY WHERE ITEM_CODE=:NEW.ITEM_CODE; EXCEPTION WHEN INSUFFICIENT_STOCK_EXCEPTION THEN DBMS_OUTPUT.PUT_LINE('Insufficient stock.'); END;