Try something like this:
SELECT pr.IT_Code, pr.ReqQty, pr.ReqDate, pr.IT_Desc, SUM(po.PO_Qty) AS Qty_Underpurchase FROM PR_DET AS pr
INNER JOIN PO_DET AS po ON po.IT_Code = pr.IT_Code
WHERE po.IT_Status = 'UnderPurchase'
GROUP BY pr.IT_Code, pr.ReqQty, pr.ReqDate, pr.IT_Desc
For Joins:
http://technet.microsoft.com/en-us/library/aa213233(v=sql.80).aspx[
^]
If you use an aggregate function (like
SUM
is one) all other columns you want to show need to be either an aggregate function too or be in the GROUP BY clause.
Edit: Fixed the SQL (was missing a ")" ;) )