为什么未在COUNT（列名）中计算空值 [英] Why aren't nulls counted in COUNT(columnname)

问题描述

所以前几天我在工作中遇到了一些问题，试图计算运行导入后有多少空值。

So I came across something the other day at work, trying to count how many null values after running an import.

所以，我做到了：

select COUNT(columnname) from table 
WHERE ColumnName is null

没有算空值...

然后我做了，

select COUNT(*) from table 
WHERE ColumnName is null

这是给我计数的。

让我烦恼的是为什么它不计算空值。

So, something that bugged me is why exactly this doesn't count the null values.

我已经看过这个问题（以及对Google的良好搜索...）：
在SQL中，count（column）和count（*）有什么区别？，
，虽然它告诉我 COUNT（列名）不算空值，但我想确切地知道为什么

I have looked at this question (along with a good search around Google...): In SQL, what's the difference between count(column) and count(*)?, and whilst it tells me that COUNT(columnname) doesn't count nulls, I would like to know exactly why nulls aren't counted using this method?

非常感谢，James
。

Many Thanks, James.

推荐答案

COUNT 对值进行计数，因为null不是一个值，因此不会被计数。

COUNT counts values, since null is not a value it does not get counted.

如果要计算所有空值，可以执行以下操作：

If you want to count all null values you could do something like this:

SELECT COUNT(ID) as NotNull, SUM(CASE WHEN ID IS NULL then 1 else 0 end) as NullCount

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