----查看max_user_connections 默认值

  MySQL> show variables like 'max_user_connections';

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

 | Variable_name        | Value |

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

 | max_user_connections |      |

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

 row in set (0.00 sec)

---设置 max_user_connections

mysql> set  @@global.max_user_connections=;

Query OK,  rows affected (0.03 sec)

mysql>  select @@max_user_connections;

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

 | @@max_user_connections |

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

 |                       |

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

  row in set (0.00 sec)

上面参数设置完后窗口，要重新登陆一下

root@dg ~]# mysql -uroot -pmysql

然后再开一个窗口登陆就会报如下错误：

root@dg ~]# mysql -uroot -pmysql

 Warning: Using a password on the command line interface can be insecure.

ERROR  (): User root already has more than 'max_user_connections' active connections

另外在登陆一个用户u2，正常登陆，但是在登陆一个u2用户，就会报错：

root@dg mysql]# mysql -uu2 -pu2

 Warning: Using a password on the command line interface can be insecure.

 Welcome to the MySQL monitor.  Commands end with ; or \g.

 Your MySQL connection id is

 Server version: 5.6.-enterprise-commercial-advanced MySQL Enterprise Server - Advanced Edition (Commercial)

 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.

max_user_connections 针对用户设计的

下面我们来看看max_connections 的作用

mysql> select @@max_connections;

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

 | @@max_connections |

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

 |                |

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

  row in set (0.00 sec)

 mysql>  set  @@global.max_connections=;

 Query OK,  rows affected (0.00 sec)

 mysql> select @@max_connections;

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

 | @@max_connections |

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

 |                  |

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

  row in set (0.00 sec)

上面参数设置完后窗口，要重新登陆一下

[root@dg mysql]# mysql -uroot -pmysql

在开一个窗口session2正常登陆：

[root@dg mysql]# mysql -uroot -pmysql

再开第三个窗口，session3 登陆的时候报错

[root@dg ~]# mysql -uroot -pmysql

 Warning: Using a password on the command line interface can be insecure.

 ERROR  (HY000): Too many connections

 [root@dg ~]# mysql -uu2 -pu2

 Warning: Using a password on the command line interface can be insecure.

 ERROR  (HY000): Too many connections

实验完毕后,改回原来的参数

mysql>  set  @@global.max_user_connections=;

 Query OK,  rows affected (0.00 sec)

 mysql>  select @@max_user_connections;

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

 | @@max_user_connections |

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

 |                       |

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

  row in set (0.00 sec)

 mysql>  set  @@global.max_connections=;

 Query OK,  rows affected (0.00 sec)

 mysql> select @@max_connections;

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

 | @@max_connections |

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

 |                |

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

  row in set (0.00 sec)

结论：

max_user_connections：限制每个用户的session连接个数，例如max_user_connections= ，那么用户u1只能连接的session数为1，如果还有用户u2，还是可以连接，但是连接数仍然为1

max_connections ：是对整个服务器的用户限制，整个服务器只能开这么多session，而不考虑用户！