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如何获取AVG列忽略NULL和零值？

我有三列来获得他们的平均值,我尝试使用以下脚本:

SELECT distinct
     AVG(cast(ISNULL(a.SecurityW,0) as bigint)) as Average1
     ,AVG(cast(ISNULL(a.TransferW,0) as bigint)) as Average2
     ,AVG(cast(ISNULL(a.StaffW,0) as bigint)) as Average3
FROM Table1 a,  Table2 b
WHERE a.SecurityW <> 0 AND a.SecurityW IS NOT NULL
AND a.TransferW<> 0 AND a.TransferWIS NOT NULL
AND a.StaffW<> 0 AND a.StaffWIS NOT NULL
AND MONTH(a.ActualTime) = 4
AND YEAR(a.ActualTime) = 2013

我没有得到任何结果,但是三列的值包括NULL和零!

无论如何在获得平均值之前排除空值？

例: AVERAGE(NOTNULL(SecurityW))