CREATE OR REPLACE TRIGGER num_in_stock_trigger
    AFTER
       INSERT ON Invoice_Details
    FOR EACH ROW
    DECLARE
       num_in_stock NUMBER;
    BEGIN

        SELECT Number_In_Stock
         INTO num_in_stock
        FROM Inventory
        WHERE Item_Id = :new.Item_Id;

        num_in_stock := num_in_stock - :new.Quantity_Ordered;

        UPDATE Inventory
           SET Number_in_Stock = num_in_stock
            WHERE Item_Id = :new.Item_Id;
    END;

  COMMIT;


  SELECT * FROM Inventory WHERE Item_Id = 12;

  INSERT INTO Invoice_Details (Order_Number, Item_id, quantity_ordered, back_ordered)
    VALUES(1,12,7,0);

  SELECT * FROM Inventory WHERE Item_Id = 12;