I had problems with the below code. For some reason, the NVL function may have given me incorrect resulting quantities. I am tearing this apart and will put it back together today. I think you are on the right track; it's just that I've never written code like the below; at least one part of it isn't working right yet.

SELECT ;
            cs.CustNo, ;
            cs.Customer, ;
            NVL(QtyIn, 000000), ;
            NVL(QtyOut, 000000), ;
            NVL(QtyIn, 000000)-NVL(QtyOut, 000000) As QtyDiff ;
         FROM Customer cs ;
         LEFT JOIN (SELECT cust, SUM(StandsIn) FROM StandsIn AS QtyIn GROUP BY 1) si ON si.cust = cs.custno ;
         LEFT JOIN (SELECT cust, SUM(SerialCnt) FROM Stands_Out AS QtyOut GROUP BY 1) so ON so.cust = cs.custno ;
         ORDER BY ;
            cs.Customer