sql server not in查询没结果原因分析

今天使用SQL Server 时，遇到使用not in 和not exist的查询结果有差异：not in 查询没结果。

原因：not in 遇到null就不工作了。

摘录：

SELECT foreignStockId
FROM[Subset].[dbo].[Products]
Probably returns a NULL. Try

SELECT  stock.IdStock
        ,stock.Descr       
FROM[Inventory].[dbo].[Stock] stock
WHERE   stock.IdStock NOTIN(SELECT foreignStockId FROM[Subset].[dbo].[Products]WHERE foreignStockId ISNOTNULL)
a NOT IN (x,y,NULL) Will always return no results as it is equivalent to

a<>




x and a<>y and a<>NULL which is

true and true and unknown

Which eva luates to unknown under the rules of three valued logic.

I normally use NOT EXISTS for this type of query

SELECT stock.idstock,
       stock.descr
FROM[Inventory].[dbo].[Stock] stock
WHERENOTEXISTS(SELECT*FROM[Subset].[dbo].[Products] p
                   WHERE  p.foreignstockid = stock.idstock)