多在哪里在MySQL查询不返回任何东西

Question

我知道这是一个非常简单的问题，但为什么不是我的查询返回任何东西？

mysql> select * from notifications; 
+----+---------+-------------+---------+--------+---------------------+ 
| id | user_id | sec_user_id | item_id | action | date    | 
+----+---------+-------------+---------+--------+---------------------+ 
| 1 |  1 |  NULL | NULL | NULL | 2015-10-09 23:47:36 | 
+----+---------+-------------+---------+--------+---------------------+ 
1 row in set (0.00 sec) 

mysql> select id from notifications where user_id = 1 and action = NULL; 
Empty set (0.00 sec) 

Answer 1

将action = null更改为action is null。

Answer 2

尝试IS NULL：

select id 
from notifications 
where user_id = 1 
and action IS NULL 

Answer 3

NULL不能等于任何东西，包括自己。你应该使用它is ..

select id from notifications where user_id = 1 and action is NULL;