where 子句中的 NULL 值

2022-01-06 00:00:00 sql null mysql

我有一张像这样的bla"表:

i've got a table "bla" like this:

[id] [name] [fk] 1 test 4 2 foo 5 3 bar NULL

如果我做sql查询

SELECT * FROM bla WHERE fk <> 4

我只得到 id 为 2 的记录.我没有得到 id 3 的记录，其中 fk 为空.我认为 NULL != 4.似乎这是错误的.

i only get the record with the id 2. i don't get the record with id 3 where fk is null. I thought NULL != 4. Seems that this is wrong.

为什么会这样?

推荐答案

NULL 不等于任何东西.您需要明确接受空值:

NULL doesn't compare equal to anything. You'll need to accept nulls explicitly:

where fk <> 4 or fk is null;

请参阅使用 NULL 了解更多信息关于 NULL 处理的信息.

See Working with NULL for more information about NULL handling.

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