MySQL查询中的多个位置不返回任何内容

Question

I know that this is a very easy question, but why isn't my query returning anything ?

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

NULL cannot be equal to anything, including itself. You should use is with it..

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

Answer 2

将action = null更改为action is null 。

Answer 3

try with IS NULL :

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