如何计算 MySQL 中的 NULL 值？
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How to count NULL values in MySQL?
提问by abbr
I want to know how can i find all the values that are NULL in the MySQL database for example I'm trying to display all the users who don't have an average yet.
我想知道如何在 MySQL 数据库中找到所有为 NULL 的值，例如我试图显示所有还没有平均值的用户。
Here is the MySQL code.
这是 MySQL 代码。
SELECT COUNT(average) as num
FROM users
WHERE user_id = '$user_id'
AND average IS_NULL
采纳答案by Hammerite
SELECT
COUNT(*) as num
FROM
users
WHERE
user_id = '$user_id' AND
average IS NULL
回答by ircmaxell
A more generic version (that doesn't depend on the where clause and hence limits your overall results):
更通用的版本（不依赖于 where 子句，因此会限制您的整体结果）：
SELECT
SUM(CASE WHEN average IS NULL THEN 1 ELSE 0 END) As null_num,
SUM(CASE WHEN average IS NOT NULL THEN 1 ELSE 0 END) AS not_null_num
FROM users
It's not better then the specific queries presented by other answers here, but it can be used in situations where using a limiting where clause is impractical (due to other information being needed)...
它并不比此处其他答案提供的特定查询更好，但可以在使用限制 where 子句不切实际的情况下使用（由于需要其他信息）...
回答by Charles Bretana
Also, you can:
此外，您还可以：
Select Count(*) - Count(Average) as NullAverages
From Users
Where user_id = '$user_id'
回答by Jacob
you're on the right track. Remove '_' from 'IS_NULL' and change 'COUNT(average)' to 'COUNT(1)' and you will have it.
你在正确的轨道上。从“IS_NULL”中删除“_”并将“COUNT(average)”更改为“COUNT(1)”，您将拥有它。
For more information on working with NULL in MYSQL see http://dev.mysql.com/doc/refman/5.0/en/working-with-null.html
有关在 MYSQL 中使用 NULL 的更多信息，请参阅http://dev.mysql.com/doc/refman/5.0/en/working-with-null.html
And for working with IS NULL specifically see
对于使用 IS NULL 的工作，请特别参阅
http://dev.mysql.com/doc/refman/5.0/en/comparison-operators.html#operator_is-null
http://dev.mysql.com/doc/refman/5.0/en/comparison-operators.html#operator_is-null
回答by μBio
I may be missing something mysql specific but this would work in sql server
我可能会遗漏一些特定于 mysql 的东西，但这可以在 sql server 中使用
SELECT COUNT(*) as num
FROM users
WHERE user_id = '$user_id'
AND average IS NULL
回答by ATorras
A bit late but you can do it in MySQL 5.0.51:
有点晚了，但你可以在 MySQL 5.0.51 中做到：
SELECT COUNT(*) AS total, COUNT(field1) AS notNullFields1
FROM table
GROUP BY field5, field6
Regards.
问候。
回答by Prozac
This is a one liner that also could be used for this problem :
这是一个也可用于解决此问题的单衬：
SELECT COUNT(IF(average IS NULL,1,0))
FROM table;
It worked like a charm for me!
它对我来说就像一个魅力！
