COUNT(*) 包括空值? [英] COUNT(*) Includes Null Values?

问题描述

MSDN 文档说明:

COUNT(*) 返回组中的项目数.这包括 NULL值和重复项.

COUNT(*) returns the number of items in a group. This includes NULL values and duplicates.

你怎么能在一个组中有一个空值?任何人都可以解释他们试图提出的观点吗?

How can you have a null value in a group? Can anyone explain the point they're trying to make?

推荐答案

如果你有这张桌子

表 1:

 Field1    Field2    Field3
 ---------------------------
   1         1         1
  NULL      NULL      NULL
   2         2        NULL
   1         3         1

然后

 SELECT COUNT(*), COUNT(Field1), COUNT(Field2), COUNT(DISTINCT Field3)
 FROM Table1

输出是:

 COUNT(*) = 4; -- count all rows, even null/duplicates

 -- count only rows without null values on that field
 COUNT(Field1) = COUNT(Field2) = 3

 COUNT(Field3) = 2 
 COUNT(DISTINCT Field3) = 1 -- Ignore duplicates

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