之前MySQL服务端本机上使用密码登陆root账号是没有问题的，但是今天不知道是动了哪里，登陆失败并有这个错误代码：
# mysql -u root -p
Enter password:
ERROR 1698 (28000): Access denied for user 'root'@'localhost'
解决步骤：
停止mysql服务
# 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 2 Server version: 5.7.10 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> use mysql
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A
Database changed
mysql> select user,plugin from user;
+------------------+-----------------------+
| user | plugin |
+------------------+-----------------------+
| root | auth_socket |
| mysql.session | mysql_native_password |
| mysql.sys | mysql_native_password |
| debian-sys-maint | mysql_native_password |
+------------------+-----------------------+
4 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)
改完之后
mysql> select user,plugin from user;
+------------------+-----------------------+
| user | plugin |
+------------------+-----------------------+
| root | mysql_native_password |
| mysql.session | mysql_native_password |
| mysql.sys | mysql_native_password |
| debian-sys-maint | mysql_native_password |
+------------------+-----------------------+
4 rows in set (0.00 sec)
重启服务，问题就解决了
root@localhost:~# service mysql restart
* Stopping MySQL database server mysqld
2019-09-23T16:01:39.843922Z mysqld_safe mysqld from pid file /var/run/mysqld/mysqld.pid ended
[ OK ]
* Starting MySQL database server mysqld No directory, logging in with HOME=/
[ OK ]
[1]+ Done mysqld_safe --skip-grant-tables
root@localhost:~# mysql -u root -p
Enter password:
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 4
Server version: 5.7.21-1ubuntu1 (Ubuntu)
