Oblicz wartość kolumny z innej kolumny w innej tabeli

Oto jak możesz to zrobić w MySQL (Twoje pytanie jest oznaczone tagami mysql i sql-server )

CREATE TRIGGER tg_bi_order_contains_items 
BEFORE INSERT ON Order_contains_items
FOR EACH ROW
  SET NEW.Ordered_price = 
  (
    SELECT Unit_price * NEW.Quantity_ordered
      FROM Item
     WHERE Item_id = NEW.Item_id
     LIMIT 1
  );

CREATE TRIGGER tg_bu_order_contains_items 
BEFORE UPDATE ON Order_contains_items
FOR EACH ROW
  SET NEW.Ordered_price = 
  (
    SELECT Unit_price * NEW.Quantity_ordered
      FROM Item
     WHERE Item_id = NEW.Item_id
     LIMIT 1
  );

Oto SQLFiddle demo