select...count()

╰半夏微凉° 2021-11-17 03:54 309阅读 0赞

declare @t table( c varchar(10));

insert @t  
select \*  
from  
(  
 select 'A' as c union all  
 select 'B' union all  
 select 'B' union all  
 select null union all  
 select null  
) t;

select count(\*) from @t;  
select count(c) from @t;  
select count(distinct c) from @t;  
select distinct c from @t;

测一下上面最后四条语句的执行结果：

![ContractedBlock.gif][] ![ExpandedBlockStart.gif][] Code   
![None.gif][]\--\---------  
![None.gif][]**5**  
![None.gif][]  
![None.gif][]  
![None.gif][]\--\---------  
![None.gif][]**3**  
![None.gif][]Warning: Null value is eliminated by an aggregate or other SET operation.  
![None.gif][]  
![None.gif][]  
![None.gif][]\--\---------  
![None.gif][]**2**  
![None.gif][]Warning: Null value is eliminated by an aggregate or other SET operation.  
![None.gif][]  
![None.gif][]  
![None.gif][]c  
![None.gif][]\--\--------  
![None.gif][]NULL  
![None.gif][]A  
![None.gif][]B  
![None.gif][]  
![None.gif][]

转载于:https://www.cnblogs.com/aspsmile/archive/2009/03/04/1402825.html

[ContractedBlock.gif]: https://www.cnblogs.com/Images/OutliningIndicators/ContractedBlock.gif
[ExpandedBlockStart.gif]: /images/20211116/d94c272e092a4e76a292b44203bc64b3.png
[None.gif]: /images/20211116/408ed8f526d143d096fcd624d4d24122.png

select...count()

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