MySQL两种删除用户语句的区别(delete user和drop user)

Z时代
2024-01-10
分类：综合

Tip：

在MySQL中，我们经常需要创建用户和删除用户，创建用户时，我们一般使用create user或者grant语句来创建，create语法创建的用户没有任何权限，需要再使用grant语法来分配权限，而grant语法创建的用户直接拥有所分配的权限。在一些测试用户创建完成之后，做完测试，可能用户的生命周期就结束了，需要将用户删除，而删除用户在MySQL中一般有两种方法，一种是drop user，另外一种是delete from mysql.user，那么这两种方法有什么区别呢？我们这里通过例子演示。

delete from mysql.user

首先，我们看看delete from mysql.user的方法。我们创建两个用户用来测试，测试环境是MySQL5.5版本，用户名分别为yeyz@'%'和yeyz@'localhost',创建用户的语法如下：


mysql 15:13:12>>create user yeyz@'%' identified by '123456';
Query OK, rows affected (. sec)
mysql 15:20:01>>grant select,create,update,delete on yeyz.yeyz to yeyz@'%';
Query OK, rows affected (. sec)
mysql 15:29:48>>GRANT USAGE ON yeyz.yeyz TO 'yeyz'@localhost IDENTIFIED BY '123456';
Query OK, rows affected (. sec)
mysql--dba_admin@127...1:(none) 15:20:39>>show grants for yeyz@'%';
+-----------------------------------------------------------------------------------------------------+
| Grants for yeyz@%                                          |
+-----------------------------------------------------------------------------------------------------+
| GRANT USAGE ON *.* TO 'yeyz'@'%' IDENTIFIED BY PASSWORD '*6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9' |
| GRANT SELECT, UPDATE, DELETE, CREATE ON `yeyz`.`yeyz` TO 'yeyz'@'%'                 |
+-----------------------------------------------------------------------------------------------------+

此时我们通过delete的方法手动删除mysql.user表中的这两个用户，在去查看用户表，我们发现：


mysql 15:20:43>>delete from mysql.user where user='yeyz';
Query OK, rows affected (. sec)
mysql 15:21:40>>select user,host from mysql.user;
+------------------+-----------------+
| user       | host      |
+------------------+-----------------+
| dba_yeyz     | localhost    |
| root       | localhost    |
| tkadmin     | localhost    |
+------------------+-----------------+
 rows in set (. sec)

已经没有这两个yeyz的用户了，此时我们使用show grants for命令查看刚才删除的用户，我们发现依旧是存在这个用户的权限说明的：

mysql 15:24:21>>show grants for yeyz@'%';

+-----------------------------------------------------------------------------------------------------+

| Grants for yeyz@% |

+-----------------------------------------------------------------------------------------------------+

| GRANT USAGE ON *.* TO 'yeyz'@'%' IDENTIFIED BY PASSWORD '*6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9' |

| GRANT SELECT, UPDATE, DELETE, CREATE ON `yeyz`.`yeyz` TO 'yeyz'@'%' |

+-----------------------------------------------------------------------------------------------------+

rows in set (0.00 sec)

说明我们虽然从mysql.user表里面删除了这个用户，但是在db表和权限表里面这个用户还是存在的，为了验证这个结论，我们重新创建一个yeyz@localhost的用户，这个用户我们只给它usage权限，其他的权限我们不配置，如下：


mysql ::>>GRANT USAGE ON yeyz.yeyz TO 'yeyz'@localhost IDENTIFIED BY '123456';
Query OK, rows affected (. sec)

这个时候，我们使用yeyz@localhost这个用户去登陆数据库服务，然后进行相关的update操作，如下：


[dba_mysql@tk-dba-mysql-stat-- ~]$ /usr/local/mysql/bin/mysql -uyeyz --socket=/data/mysql_4306/tmp/mysql.sock --port= -p -hlocalhost
Enter password: 
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 
Server version: 5.5.-log MySQL Community Server (GPL)
Copyright (c) , , Oracle and/or its affiliates. All rights reserved.
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
mysql--yeyz@localhost:(none) 15:31:05>>select * from yeyz.yeyz;
+------+
| id  |
+------+
|  3 |
|  4 |
|  5 |
+------+
 rows in set (. sec)
mysql--yeyz@localhost:(none) 15:31:16>>delete from yeyz.yeyz where id=;
Query OK, row affected (. sec)
mysql--yeyz@localhost:(none) 15:31:32>>select * from yeyz.yeyz;
+------+
| id  |
+------+
|  3 |
|  4 |
+------+
 rows in set (. sec)

最终出现的结果可想而知，一个usage权限的用户，对数据库总的表进行了update操作，而且还成功了。这一切得益于我们delete from mysql.user的操作，这种操作虽然从user表里面删除了记录，但是当这条记录的host是%时，如果重新创建一个同名的新用户，此时新用户将会继承以前的用户权限，从而使得用户权限控制失效，这是很危险的操作，尽量不要执行。

再开看看drop的方法删除用户

首先，我们删除掉刚才的那两个用户，然后使用show grants for语句查看他们的权限：


mysql ::>>drop user yeyz@'%';
Query OK, rows affected (0.00 sec)
mysql ::>>drop user yeyz@'localhost';
Query OK, rows affected (0.00 sec)
mysql ::>>
mysql ::>>show grants for yeyz@'%';
ERROR (): There is no such grant defined for user 'yeyz' on host '%'
mysql ::>>show grants for yeyz@'localhost';
ERROR (): There is no such grant defined for user 'yeyz' on host '192.168.18.%'

可以看到，权限已经完全删除了，此时我们重新创建一个只有select权限的用户：


mysql ::>>GRANT SELECT ON *.* TO 'yeyz'@'localhost' IDENTIFIED BY PASSWORD '*6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9';
Query OK, rows affected (. sec)

我们使用这个用户登录到数据库服务，然后尝试进行select、update以及create操作，结果如下：


[dba_mysql@tk-dba-mysql-stat-10-104 ~]$ /usr/local/mysql/bin/mysql -uyeyz --socket=/data/mysql_4306/tmp/mysql.sock --port=4306 -p -hlocalhost
Enter password: 
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 
Server version: 5.5.19-log MySQL Community Server (GPL)
Copyright (c) , , Oracle and/or its affiliates. All rights reserved.
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
mysql ::>>select * from yeyz.yeyz;
+------+
| id  |
+------+
|   |
|   |
|   |
+------+
 rows in set (0.00 sec)
mysql ::>>update yeyz.yeyz set id= where id=;
ERROR (): UPDATE command denied to user 'yeyz'@'localhost' for table 'yeyz'
mysql ::>>create table test (id int);
ERROR (D000): No database selected
mysql ::>>create table yeyz.test (id int);
ERROR (): CREATE command denied to user 'yeyz'@'localhost' for table 'test'