where子句中的NULL值

问题描述

我有一个这样的表"bla":

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

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

如果我执行sql查询

SELECT * FROM bla WHERE fk <> 4

我只获得ID为2的记录.我没有获得ID为3的记录，其中fk为空.我以为NULL！=4.看来这是错误的.

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null:

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

where fk <> 4 or fk is null;

有关更多信息，请参见使用NULL 有关NULL处理的信息.

See Working with NULL for more information about NULL handling.

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