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