----查看max_user_connections 默认值  MySQL> show variables like ‘max_user_connections‘;+----------------------+-------+ | Variable_name        | Value | +----------------------+-------+ | max_user_connections | 0     | +----------------------+-------+1 row in set (0.00 sec)---设置 max_user_connectionsmysql> set  @@global.max_user_connections=1;Query OK, 0 rows affected (0.03 sec)mysql>  select @@max_user_connections; +------------------------+ | @@max_user_connections | +------------------------+ |                      1 | +------------------------+ 1 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 1203 (42000): 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 147 Server version: 5.6.27-enterprise-commercial-advanced MySQL Enterprise Server - Advanced Edition (Commercial) Copyright (c) 2000, 2015, 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 | +-------------------+ |               151 | +-------------------+ 1 row in set (0.00 sec) mysql>  set  @@global.max_connections=2; Query OK, 0 rows affected (0.00 sec) mysql> select @@max_connections; +-------------------+ | @@max_connections | +-------------------+ |                 2 | +-------------------+ 1 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 1040 (HY000): Too many connections [root@dg ~]# mysql -uu2 -pu2 Warning: Using a password on the command line interface can be insecure. ERROR 1040 (HY000): Too many connections实验完毕后,改回原来的参数mysql>  set  @@global.max_user_connections=0; Query OK, 0 rows affected (0.00 sec) mysql>  select @@max_user_connections; +------------------------+ | @@max_user_connections | +------------------------+ |                      0 | +------------------------+ 1 row in set (0.00 sec) mysql>  set  @@global.max_connections=200; Query OK, 0 rows affected (0.00 sec) mysql> select @@max_connections; +-------------------+ | @@max_connections | +-------------------+ |               200 | +-------------------+ 1 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=1 ，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1max_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！