just a thought....it may help explain 

SELECT c.id_product,
   SUM(COALESCE(c.received_qty, c.ordered_qty)) AS Total,
   c.id_cart,
   p.id_purchase,
   p.id_department
FROM Cart_Item AS c
  LEFT OUTER JOIN Purchase AS p ON c.id_cart = p.id_cart
GROUP BY c.id_product,
   c.id_cart,
   p.id_purchase,
   p.id_department;