之前MySQL服务端本机上使用密码登陆root账号是没有问题的，但是今天不知道是因为动了哪里，登陆失败并有这个错误代码：
~$ mysql -u root -p
Enter password:
ERROR (): Access denied for user 'root'@'localhost'
解决步骤：
停止mysql服务
~$ sudo service mysql stop
以安全模式启动MySQL
~$ sudo mysqld_safe --skip-grant-tables &
MySQL启动之后就可以不用密码登陆了
~$ mysql -u root
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is
Server version: 5.7. MySQL Community Server (GPL)
查看一下user表，错误的起因就是在这里， root的plugin被修改成了auth_socket，用密码登陆的plugin应该是mysql_native_password。
mysql> select user, plugin from mysql.user;
+-----------+-----------------------+
| user | plugin |
+-----------+-----------------------+
| root | auth_socket |
| mysql.sys | mysql_native_password |
| dev | mysql_native_password |
+-----------+-----------------------+
3 rows in set (0.01 sec)
关于auth_socket，在官方有说明： https://dev.mysql.com/doc/mysql-security-excerpt/5.5/en/socket-authentication-plugin.html ，反正现在暂时不用它， 那就把这里改了。
mysql> update mysql.user set authentication_string=PASSWORD('newPwd'), plugin='mysql_native_password' where user='root';
Query OK, 1 row affected, 1 warning (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 1
mysql> flush privileges;
Query OK, 0 rows affected (0.00 sec)
重启服务，问题就解决了
~$ sudo service mysql stop
...
* MySQL Community Server 5.7. is stopped
~$ sudo service mysql start
..
* MySQL Community Server 5.7. is started
~$ mysql -u root -p
Enter password:
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is
Server version: 5.7. MySQL Community Server (GPL)