mssql null µÄÎÊÌâ - MS-SQL Server / ÒÉÄÑÎÊÌâ

select case null
when '' then '1' 
when null then '2' --ΪʲôûÓÐÔÚÕâ¸öλÖÃÅжϳöÀ´
else '3' 
end a 

select 1 a,2 b into #a
select isnull(b,0) b from #a where a=2
--bÁÐûÓÐÊý¾Ý Ó¦¸ÃÓÃʲôÀ´ÅжÏbÊÇ¿ÕµÄÄØ

---

--bÁÐûÓÐÊý¾Ý Ó¦¸ÃÓÃʲôÀ´ÅжÏbÊÇ¿ÕµÄÄØ
where b is null

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NULL Óë NULL ±ÈĬÈÏÊÇUNKWONEµÄ,

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SQL code:

DECLARE @b INT
SELECT @b = ISNULL(b,0) from #a WHERE a=2
IF @@ROWCOUNT = 0
SELECT @b = 0
SELECT @b AS b

---

1¡¢
 null is null nul <> null
2¡¢¿Õ¼¯ºÏûÓÐÊôÐÔ

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isnull(b,0)µÄÒâ˼¾ÍÊÇbûÓÐÊý¾ÝµÄʱºò·µ»á0Âï

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NULL Óë NULL ±ÈĬÈÏÊÇUNKWONEµÄ,

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