CREATE OR REPLACE TRIGGER ALST0001
    AFTER
    INSERT OR UPDATE OR DELETE ON STYLES
    FOR EACH ROW
DECLARE
 L_QTY  NUMBER(10);
BEGIN
   IF INSERTING THEN
      L_QTY:=:NEW.QTY;
      UPDATE PURCHASE_ORDERS
      SET QTY = QTY + L_QTY
      WHERE ORDERNO = :NEW.ORDERNO; /**OR WHATEVER THE FEILDS ON WHICH YOU HAVE A RELATION BETEWEEN YOUR TABLES**/
   ELSIF UPDATING THEN
      L_QTY:=:NEW.QTY - :OLD.QTY; /**IF YOU AR INCREASING QTY IN STYLES THEN THIS +VE IF DECREASING THEN -VE **/
      UPDATE PURCHASE_ORDERS
      SET QTY = QTY + L_QTY
      WHERE ORDERNO = :NEW.ORDERNO; /**OR WHATEVER THE FEILDS ON WHICH YOU HAVE A RELATION BETEWEEN YOUR TABLES**/
   ELSE
      L_QTY:=:OLD.QTY;
      UPDATE PURCHASE_ORDERS
      SET QTY = QTY - L_QTY
      WHERE ORDERNO = :NEW.ORDERNO; /**OR WHATEVER THE FEILDS ON WHICH YOU HAVE A RELATION BETEWEEN YOUR TABLES**/
   END IF;
END;