mysql 用户变量不能赋予null值

查询的user_id 不为null

mysql> SELECT  @userId :=user_id  FROM ta_user_info WHERE account_name = '13870957087';
SELECT IF(@userId IS NOT NULL , 1,0);
SELECT @userId;

结果：

+---------------------+
| @userId :=user_id   |
+---------------------+
| 5000000000001173892 |
+---------------------+
1 row in set

+-------------------------------+
| IF(@userId IS NOT NULL , 1,0) |
+-------------------------------+
|                             1 |
+-------------------------------+
1 row in set

+---------------------+
| @userId             |
+---------------------+
| 5000000000001173892 |
+---------------------+
1 row in set

从结果可以看出用户变量@userId获得了查询结果，并通过if函数判断不为null

查询的user_id 为null

mysql> SELECT  @userId :=user_id  FROM ta_user_info WHERE account_name = 'xxxx';
SELECT IF(@userId IS NOT NULL , 1,0);
SELECT @userId;

结果：

Empty set

+-------------------------------+
| IF(@userId IS NOT NULL , 1,0) |
+-------------------------------+
|                             1 |
+-------------------------------+
1 row in set

+---------------------+
| @userId             |
+---------------------+
| 5000000000001173892 |
+---------------------+
1 row in set

发现用户变量@userId确和我们想的不一样，而是上次查询的结果只值，而且if函数也不是null。推测是null不能赋值给用户变量

3.查询的user_id 不为null，换成了其他非null的值

mysql> SELECT  @userId :=user_id  FROM ta_user_info WHERE account_name = '490454361@qq.com';
SELECT IF(@userId IS NOT NULL , 1,0);
SELECT @userId;

结果：

+---------------------+
| @userId :=user_id   |
+---------------------+
| 5889529241746227315 |
+---------------------+
1 row in set

+-------------------------------+
| IF(@userId IS NOT NULL , 1,0) |
+-------------------------------+
|                             1 |
+-------------------------------+
1 row in set

+---------------------+
| @userId             |
+---------------------+
| 5889529241746227315 |
+---------------------+
1 row in set

可以看出用户变量@userId又被赋予了新的值。说明第二步查询的推论是正确的，即null不能赋值给用户变量

所以想通过用户变量使用if函数判断查询的userId是否为null 的方法也不可行。