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最近在做 MySQL 多实例的时候，采用不同的方式登录 MySQL 数据库，发现 mysql -uroot -p -h 127.0.0.1 -P3308，登录时居然不要密码就可以登录 
，吓得我一身汗，经过检查终于找到原因记录下来，给小白们参考。
[root@mysql ~]# mysql -uroot -p -S /data/3308/mysql.sock
Enter password: #需要输入密码
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 23
Server version: 5.5.32-log Source distribution
Copyright (c) 2000, 2013, 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 current_user();
+—————-+
| current_user() |
+—————-+
| root@localhost |
+—————-+
1 row in set (0.00 sec)
******************************************************************
[root@mysql ~]# mysql -uroot -p -h 127.0.0.1 -P3308
Enter password: #不需要输入密码，直接回车登录
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 24
Server version: 5.5.32-log Source distribution
Copyright (c) 2000, 2013, 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>
mysql> select current_user();
+—————-+
| current_user() |
+—————-+
| root@127.0.0.1 |
+—————-+
1 row in set (0.00 sec)
不要密码登录原因：
登录数据库查看密码
mysql> select user,host,password from mysql.user;
+——+———–+——————————————-+
| user | host | password |
+——+———–+——————————————-+
| root | localhost | *FE28814B4A8B3309DAC6ED7D3237ADED6DA1E515 |
| root | mysql | |
| root | 127.0.0.1 | |
| root | ::1 | |
| | localhost | |
| | mysql | |
| tom | % | *71FF744436C7EA1B954F6276121DB5D2BF68FC07 |
| tom1 | % | *36B46F2410B8A8E6E1DC6952C5C26929FDC07421 |
| tom2 | % | *FEF6FE64F2588DC11E69F0DC5A704E231D1AAEFC |
| tom | localhost | *71FF744436C7EA1B954F6276121DB5D2BF68FC07 |
+——+———–+——————————————-+
10 rows in set (0.00 sec)
总结：第一种方式登录后，用户是 root@localhost，第二种方式登录后，用户是 root@127.0.0.1 查看数据库，发现 root@localhost 没有设置密码，所以使用第一种方式登录不需要输入密码。
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