hi, try this

[sql]SELECT B.StockName, B.Total-ISNULL(S.Total,0) AS Total
FROM
(SELECT StockName, SUM(Amount) AS Total FROM Buy GROUP BY StockName) B
LEFT JOIN
(SELECT StockName, SUM(Amount) AS Total FROM Sell GROUP BY StockName) S
ON B.StockName=S.StockName[/sql]