Calculating the difference between result columns

Hi All,

I'm stuck again.  I have written two queries.  The first calculates the total
quantity of items that have been ordered.  The second calculates the total
quantity of items that have been received.  Now I want to find the difference
which will tell me what is still on order.  Any hints would be appreciated.

SELECT purchase_order.tbl_line_item.item_id,
       sum( purchase_order.tbl_line_item.quantity ) AS ordered
  FROM purchase_order.tbl_line_item
  JOIN purchase_order.tbl_detail
 USING (po_number)
 WHERE NOT purchase_order.tbl_detail.closed
 GROUP BY purchase_order.tbl_line_item.item_id;

SELECT purchase_order.tbl_line_item.item_id,
       sum(purchase_order.tbl_receiving.quantity) AS received
  FROM purchase_order.tbl_line_item
  JOIN purchase_order.tbl_receiving
    ON ( purchase_order.tbl_line_item.po_number =
purchase_order.tbl_receiving.po_number AND
         purchase_order.tbl_line_item.po_line =
purchase_order.tbl_receiving.po_line )
  JOIN purchase_order.tbl_detail
    ON ( purchase_order.tbl_line_item.po_number =
purchase_order.tbl_detail.po_number)
 WHERE NOT purchase_order.tbl_detail.closed
 GROUP BY purchase_order.tbl_line_item.item_id;

Kind Regards,
Keith

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99main Internet Services http://www.99main.com