一、试验基础环境配置
1、查看当前数据库的最大连接数
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)
2、设置全局变量max_connections为3(基于试验目的，做测试)
mysql> set global max_connections=3;
Query OK,0 rows affected (0.01 sec)
3、查看设置是否生效.max_connections已经设置为3.
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 |3 |
| max_user_connections | 0 |
+--------------------------+-----------------+
7 rows in set (0.00 sec)
4、退出当前登陆，再次登陆查看是否生效.
mysql> exitBye
C:\Users\ArcerZhang>mysql -uroot -parcerzhang
Welcometo the MySQL monitor. Commands end with ; or\g.
Your MySQL connection idis 3Server version:5.5.28MySQL Community Server (GPL)
Copyright (c)2000, 2012, Oracle and/or its affiliates. Allrights reserved.
Oracleis a registered trademark of Oracle Corporation and/orits
affiliates. Other names may be trademarksoftheir respective
owners.
Type'help;' or '\h' for help. Type '\c' to clear the currentinput statement.
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 | 3 |
| max_user_connections | 0 |
+--------------------------+-----------------+
7 rows in set (0.00 sec)
二、查看当前数据库连接数
mysql> usearcerzhangdb;Databasechanged
mysql>show processlist;+----+------+-----------------+--------------+---------+------+-------+------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+------+-----------------+--------------+---------+------+-------+------------------+
| 3 | root | localhost:53232 | arcerzhangdb | Query | 0 | NULL | show processlist |
+----+------+-----------------+--------------+---------+------+-------+------------------+
1 row in set (0.00 sec)
三、分别打开第二个、第三个、第四个、第五个CMD窗口登录数据库
C:\Users\ArcerZhang>mysql -uroot -parcerzhang
Welcometo the MySQL monitor. Commands end with ; or\g.
Your MySQL connection idis 4Server version:5.5.28MySQL Community Server (GPL)
Copyright (c)2000, 2012, Oracle and/or its affiliates. Allrights reserved.
Oracleis a registered trademark of Oracle Corporation and/orits
affiliates. Other names may be trademarksoftheir respective
owners.
Type'help;' or '\h' for help. Type '\c' to clear the currentinput statement.
mysql> usearcerzhangdb;Databasechanged
mysql>show tables;+------------------------+
| Tables_in_arcerzhangdb |
+------------------------+
| inno_tab |
| t |
| t_myisam |
| tablespace_test |
| tt |
| ttt |
+------------------------+
6 rows in set (0.00sec)
mysql> desct;+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | 0 | |
| name | varchar(10) | YES | | NULL | |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.12 sec)
打开第二个窗口
mysql>show processlist;+----+------+-----------------+--------------+---------+------+-------+------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+------+-----------------+--------------+---------+------+-------+------------------+
| 3 | root | localhost:53232 | arcerzhangdb | Query | 0 | NULL | show processlist |
| 4 | root | localhost:53251 | arcerzhangdb | Sleep | 6 | | NULL |
+----+------+-----------------+--------------+---------+------+-------+------------------+
2 rows in set (0.00 sec)
查看数据库连接
C:\Users\ArcerZhang>mysql -uroot -parcerzhang
Welcometo the MySQL monitor. Commands end with ; or\g.
Your MySQL connection idis 5Server version:5.5.28MySQL Community Server (GPL)
Copyright (c)2000, 2012, Oracle and/or its affiliates. Allrights reserved.
Oracleis a registered trademark of Oracle Corporation and/orits
affiliates. Other names may be trademarksoftheir respective
owners.
Type'help;' or '\h' for help. Type '\c' to clear the currentinput statement.
mysql> usearcerzhangdb;Databasechanged
mysql> select * fromtt;+------+------+
| id | msg |
+------+------+
| 1 | a |
| 2 | b |
| 3 | c |
| 4 | d |
| 5 | e |
| 1000 | f |
+------+------+
6 rows in set (0.06sec)
mysql>
打开第三个窗口
mysql>show processlist;+----+------+-----------------+--------------+---------+------+-------+------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+------+-----------------+--------------+---------+------+-------+------------------+
| 3 | root | localhost:53232 | arcerzhangdb | Query | 0 | NULL | show processlist |
| 4 | root | localhost:53251 | arcerzhangdb | Sleep | 43 | | NULL |
| 5 | root | localhost:53264 | arcerzhangdb | Sleep | 5 | | NULL |
+----+------+-----------------+--------------+---------+------+-------+------------------+
3 rows in set (0.00 sec)
查看数据库连接
C:\Users\ArcerZhang>mysql -uroot -parcerzhang
Welcometo the MySQL monitor. Commands end with ; or\g.
Your MySQL connection idis 6Server version:5.5.28MySQL Community Server (GPL)
Copyright (c)2000, 2012, Oracle and/or its affiliates. Allrights reserved.
Oracleis a registered trademark of Oracle Corporation and/orits
affiliates. Other names may be trademarksoftheir respective
owners.
Type'help;' or '\h' for help. Type '\c' to clear the currentinput statement.
mysql> usearcerzhangdb;Databasechanged
mysql>show tables;+------------------------+
| Tables_in_arcerzhangdb |
+------------------------+
| inno_tab |
| t |
| t_myisam |
| tablespace_test |
| tt |
| ttt |
+------------------------+
6 rows in set (0.00sec)
mysql> select * fromttt;+----+------+
| id | msg |
+----+------+
| 1 | a |
+----+------+
1 row in set (0.07sec)
mysql>
打开第四个窗口
mysql>show processlist;+----+------+-----------------+--------------+---------+------+-------+------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+------+-----------------+--------------+---------+------+-------+------------------+
| 3 | root | localhost:53232 | arcerzhangdb | Query | 0 | NULL | show processlist |
| 4 | root | localhost:53251 | arcerzhangdb | Sleep | 97 | | NULL |
| 5 | root | localhost:53264 | arcerzhangdb | Sleep | 59 | | NULL |
| 6 | root | localhost:53274 | arcerzhangdb | Sleep | 5 | | NULL |
+----+------+-----------------+--------------+---------+------+-------+------------------+
4 rows in set (0.00 sec)
查看数据库连接
Microsoft Windows [版本 10.0.10586](c)2015Microsoft Corporation。保留所有权利。
C:\Users\ArcerZhang>mysql -uroot -parcerzhang
ERROR1040(HY000): Too many connections
C:\Users\ArcerZhang>
打开第五个窗口
四、结论
第一个连接窗口为初始窗口，或者说叫做监控窗口.
第二个连接窗口应该算是我们设置max_connections=3后第一个真正意义上的连接用户.
第三个连接窗口应该算是我们设置max_connections=3后第二个真正意义上的连接用户.
第四个连接窗口应该算是我们设置max_connections=3后第三个真正意义上的连接用户.
所以当打开第五个窗口的时候，会报如下错误:
C:\Users\ArcerZhang>mysql -uroot -parcerzhang
ERROR1040 (HY000): Too many connections
