sql - COUNT(*) 包括空值？

Question

MSDN文件指出:

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？谁能解释他们试图提出的观点？

Answer 1

如果你有这张 table

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