不使用-h参数来指定登录host，默认会连接localhost，仅当mysql.user表中有一条对应的localhost访问授权(username@%不对任何主机做限制也不行)时登录才成功，否则登录会被拒绝。
虚拟机VMUest上安装两个MySQL实例，两个实例搭建了Master(端口3306)-Slave(端口3307)，主从数据完全一致。
mysql> select Host,User,Password from mysql.user; +-----------------------+--------+-------------------------------------------+ | Host | User | Password | +-----------------------+--------+-------------------------------------------+ | localhost | root | *6B4F89A54E2D27ECD7E8DA05B4AB8FD9D1D8B119 | | localhost.localdomain | root | | | 127.0.0.1 | root | | | ::1 | root | | | localhost | | | | localhost.localdomain | | | | % | mydba | *80BF8C1F4008F25267DB194E29D2E8BC20C836ED | | % | backup | *975B2CD4FF9AE554FE8AD33168FBFC326D2021DD | | 192.168.85.% | repl | *A424E797037BF97C19A2E88CF7891C5C2038C039 | +-----------------------+--------+-------------------------------------------+ 9 rows in set
使用root用户登录
#不指定-h参数，默认就会走localhost [uest@VMUest ~]$ mysql -uroot -p Enter password: Welcome to the MySQL monitor. Commands end with ; or g. Your MySQL connection id is 3 Server version: 5.6.35-log Source distribution Copyright (c) 2000, 2016, 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 @@port; +--------+ | @@port | +--------+ | 3306 | +--------+ 1 row in set (0.00 sec) mysql> exit Bye [uest@VMUest ~]$ mysql -uroot -p -P3307 Enter password: Welcome to the MySQL monitor. Commands end with ; or g. Your MySQL connection id is 4 Server version: 5.6.35-log Source distribution Copyright (c) 2000, 2016, 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 @@port; +--------+ | @@port | +--------+ | 3306 | +--------+ 1 row in set (0.00 sec) mysql> exit Bye #指定-h+ip参考，同时指定-P端口 [uest@VMUest ~]$ mysql -uroot -p -h127.0.0.1 -P3306 Enter password: Welcome to the MySQL monitor. Commands end with ; or g. Your MySQL connection id is 5 Server version: 5.6.35-log Source distribution Copyright (c) 2000, 2016, 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 @@port; +--------+ | @@port | +--------+ | 3306 | +--------+ 1 row in set (0.00 sec) mysql> exit Bye [uest@VMUest ~]$ mysql -uroot -p -h127.0.0.1 -P3307 Enter password: Welcome to the MySQL monitor. Commands end with ; or g. Your MySQL connection id is 3 Server version: 5.6.35-log Source distribution Copyright (c) 2000, 2016, 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 @@port; +--------+ | @@port | +--------+ | 3307 | +--------+ 1 row in set (0.00 sec) mysql> exit Bye #指定-h+localhost参考，同时指定-P端口 [uest@VMUest ~]$ mysql -uroot -p -hlocalhost -P3307 Enter password: Welcome to the MySQL monitor. Commands end with ; or g. Your MySQL connection id is 10 Server version: 5.6.35-log Source distribution Copyright (c) 2000, 2016, 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 @@port; +--------+ | @@port | +--------+ | 3306 | +--------+ 1 row in set (0.00 sec) mysql> exit Bye [uest@VMUest ~]$
从上面的结果可以看出只有在指定-h+ip，同时指定-P端口的情况下，才能连接到指定的端口中。localhost实际就是指定通过socket连接！！！
注意：我们的mysql.user表中root@'127.0.0.1'对应的Password为空(即不需要输入密码)，但实际登录过程需要输入密码。这是受skip_name_resolve参数影响
mysql> show variables like '%skip_name_resolve%'; +-------------------+-------+ | Variable_name | Value | +-------------------+-------+ | skip_name_resolve | OFF | +-------------------+-------+ 1 row in set
该参数的值为OFF，root@'127.0.0.1'会转化为root@'localhost'登录，它使用的是root@'localhost'定义的密码。可尝试开启skip_name_resolve参数(配置文件my.cnf中添加skip_name_resolve=1)，然后使用空密码登录。
mysql.user表中第5条记录Host='localhost'，User和Password字段为空，也就是localhost可以不指定用户、密码直接登录
[uest@VMUest ~]$ mysql Welcome to the MySQL monitor. Commands end with ; or g. Your MySQL connection id is 26 Server version: 5.6.35-log Source distribution Copyright (c) 2000, 2016, 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 @@port; +--------+ | @@port | +--------+ | 3306 | +--------+ 1 row in set (0.00 sec) mysql>
还是有一些小细节需要注意。引用iVictor一段话：'t1'@'%'中包含't1'@'127.0.0.1'，如果开启skip_name_resolve参数，则't1'@'%'中定义的密码可用于't1'@'127.0.0.1'的登录；如果没有开启该参数，则't1'@'127.0.0.1'会转化为't1'@'localhost'登录，如果用户表存在't1'@'localhost'条目，根据最精确匹配原则(Host->User)，需要使用't1'@'localhost'对应的密码登录；如果用户表不存在't1'@'localhost'条目，'t1'@'%'定义的密码依旧适用。
从库开启skip_name_resolve参数，从库可以使用 mysql -h127.0.0.1 -P3307 -umydba -p 登录，对应user表中条目mydba@'%'；主库没有开启skip_name_resolve参数，它如果尝试使用 mysql -h127.0.0.1 -P3306 -umydba -p 登录，会转化为mydba@'localhost'，user表中没有此条目，登录拒绝！
MySQL存放于内存结构中的权限信息何时被更新：
FLUSH PRIVILEGES会强行让MySQL更新Load到内存中的权限信息；GRANT、REVOKE或者CREATE USER和DROP USER操作会直接更新内存中的权限信息；重启MySQL会让MySQL完全从grant tables中读取权限信息。
内存结构中的权限信息更新之后对已经连接上的用户何时生效：
对于Global Level的权限信息的修改，仅仅只有更改之后新建连接才会用到，对于已经连接上的session并不会受到影响。
对于Database Level的权限信息的修改，只有当客户端请求执行了“use database_name”命令之后，才会在重新校验中使用到新的权限信息。
对于Table Level和Column Level的权限，在下一次需要使用到该权限的Query被请求的时候生效。