MySQL如何回收用户的权限？

Question

有个用户 lwx，查看user表如下：

mysql> select user,host from mysql.user;
+---------------+--------------+
| user          | host         |
+---------------+--------------+
| lwx           | %            |
| qydpw         | %            |
| root          | %            |
| root          | 127.0.0.1    |
| zentao        | 127.0.0.1    |
| root          | ::1          |
| root          | lwx          |
| mysql.session | localhost    |
| mysql.sys     | localhost    |
| qppp          | localhost    |
| root          | localhost    |
+---------------+--------------+
11 rows in set (0.00 sec)

查看下这个用户的权限，如下：

mysql> show grants for 'lwx'@'%';
+--------------------------------------------------------------------------+
| Grants for lwx@%                                                |
+--------------------------------------------------------------------------+
| GRANT USAGE ON *.* TO 'lwx'@'%'                                 |
| GRANT SELECT ON `test`.`student` TO 'lwx'@'%' WITH GRANT OPTION |
+--------------------------------------------------------------------------+
2 rows in set (0.00 sec)

我现在想回收GRANT USAGE ON . TO 'lwx'@'%'，输入下面的语句：

mysql> revoke usage on *.* from 'lwx'@'%';
Query OK, 0 rows affected (0.00 sec)

mysql> flush privileges;
Query OK, 0 rows affected (0.00 sec)

mysql> show grants for 'lwx'@'%';
+--------------------------------------------------------------------------+
| Grants for lwx@%                                                |
+--------------------------------------------------------------------------+
| GRANT USAGE ON *.* TO 'lwx'@'%'                                 |
| GRANT SELECT ON `test`.`student` TO 'lwx'@'%' WITH GRANT OPTION |
+--------------------------------------------------------------------------+
2 rows in set (0.00 sec)

很明显，无法回收，为什么？

另外，权限GRANT SELECT ON test.student TO 'lwx'@'%' WITH GRANT OPTION，这个权限又如何回收呢？