之前MySQL服務端本機上使用密碼登陸root賬號是沒有問題的，但是今天不知道是因為動了哪裡，登陸失敗並有這個錯誤程式碼：

~$ mysql -u root -p
Enter password: 
ERROR 1698 (28000): Access denied for user 'root'@'localhost'

解決步驟：

停止mysql服務

~$ sudo service mysql stop

以安全模式啟動MySQL

~$ sudo mysqld_safe --skip-grant-tables &

MySQL啟動之後就可以不用密碼登陸了

~$ 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)

檢視一下user表，錯誤的起因就是在這裡， root的plugin被修改成了auth_socket，用密碼登陸的plugin應該是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)

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)

重啟服務，問題就解決了

~$ sudo service mysql stop
...
 * MySQL Community Server 5.7.10 is stopped
~$ sudo service mysql start
..
 * MySQL Community Server 5.7.10 is started
~$ mysql -u root -p
Enter password: 
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)