SQL COUNT(*) 包括空值？
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时间：2020-09-01 05:00:13 来源:igfitidea点击:
COUNT(*) Includes Null Values?
提问by Derrick Moeller
回答by Juan Carlos Oropeza
If you have this table
如果你有这张桌子
Table1:
表格1：
Field1 Field2 Field3
---------------------------
1 1 1
NULL NULL NULL
2 2 NULL
1 3 1
Then
然后
SELECT COUNT(*), COUNT(Field1), COUNT(Field2), COUNT(DISTINCT Field3)
FROM Table1
Output Is:
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
回答by ACon
You can have a Null value in a row depending on how you do your joins. Be sure you are counting the right field or correcting your joins.
根据您的连接方式，您可以在一行中有一个 Null 值。确保您正在计算正确的字段或更正您的连接。
