对一列求和，然后减去另一列的总和

我有一个包含以下字段的表：

EmpID,
Code,
Amount,
TransDate,
CM,
CMDate

我想要访问数据网格的是SUM一切Amount具有相同的Code and SUM the CM具有相同的Code as the Amount然后减去SUM(Amount) to the SUM(CM)但如果结果是NULL（例如，没有CM求和并减去SUM(Amount)）然后只需选择SUM(Amount)。我想出了这个声明：

SELECT
    TransDate, 
    Code, 
    SUM(Amount) - SUM(CM) AS 'Result'
FROM DeductionTbl
WHERE 
    (EmpID = @empID)
GROUP BY Code, TransDate
ORDER BY TransDate

如果有的话我会得到结果值SUM(CM)减去到SUM(Amount)但我得到了结果NULL如果没有则值SUM(CM)一样的Code。我不想要NULL数据网格上的值。如果有一个NULL值只是返回SUM(Amount)。有没有办法做到这一点？谢谢！

use ISNULLs:

SELECT
    TransDate, 
    Code, 
    isnull(SUM(isnull(Amount,0)),0) - isnull(SUM(isnull(CM,0)),0) AS 'Result'
FROM DeductionTbl
WHERE 
    (EmpID = @empID)
GROUP BY Code, TransDate
ORDER BY TransDate

NOTE:

The SUM(ISNULL(CM,0))可能会回来NULL当运行查询时 空结果集也避免了ANSI WARNINGS about aggregating NULL.

And ISNULL(SUM(CM),0)不被视为可为空。