--查看当前max_user_connections连接参数值为0，表示没有限制.mysql> show global variables like ‘%connect%‘;
+--------------------------+-----------------+
| Variable_name            | Value           |
+--------------------------+-----------------+
| character_set_connection | utf8            |
| collation_connection     | utf8_general_ci |
| connect_timeout          | 10              |
| init_connect             |                 |
| max_connect_errors       | 10              |
| max_connections          | 160             |
| max_user_connections     | 0               |
+--------------------------+-----------------+
7 rows in set (0.00 sec)
--设置max_user_connections值为2
mysql> set global max_user_connections=2;
Query OK, 0 rows affected (0.00 sec)
--查看设置是否生效
mysql> show global variables like ‘%connect%‘;
+--------------------------+-----------------+
| Variable_name            | Value           |
+--------------------------+-----------------+
| character_set_connection | utf8            |
| collation_connection     | utf8_general_ci |
| connect_timeout          | 10              |
| init_connect             |                 |
| max_connect_errors       | 10              |
| max_connections          | 160             |
| max_user_connections     | 2               |
+--------------------------+-----------------+
7 rows in set (0.00 sec)
--退出数据库
mysql> exit
Bye
--登录数据库
C:\Users\ArcerZhang>mysql -uroot -parcerzhang
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 7
Server version: 5.5.28 MySQL Community Server (GPL)

Copyright (c) 2000, 2012, 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.
--查看数据库
mysql> show global variables like ‘%connect%‘;
+--------------------------+-----------------+
| Variable_name            | Value           |
+--------------------------+-----------------+
| character_set_connection | utf8            |
| collation_connection     | utf8_general_ci |
| connect_timeout          | 10              |
| init_connect             |                 |
| max_connect_errors       | 10              |
| max_connections          | 160             |
| max_user_connections     | 2               |
+--------------------------+-----------------+
7 rows in set (0.00 sec)

mysql>

二、登录第一个窗口

C:\Users\ArcerZhang>mysql -uroot -parcerzhang
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 8
Server version: 5.5.28 MySQL Community Server (GPL)

Copyright (c) 2000, 2012, 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>

三、登录第二个窗口

C:\Users\ArcerZhang>mysql -uroot -parcerzhang
ERROR 1203 (42000): User root already has more than ‘max_user_connections‘ active connections

C:\Users\ArcerZhang>

四、结论

当max_user_connections设置为2时，使用一个用户登录mysql server，当开启第二窗口登录时，就会报错了.而且第一个试验窗口也算是一个用户登录.