ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using password: NO)

　　在安装好的MySQL服务器上，配置了环境变量之后，发现用mysql无法登录，报如题的错误，实在没有办法，决定用安全模式对root用户修改密码：

　　首先关闭正在运行的MySQL;

　　在一个终端窗口运行命令：mysqld_safe --user=mysql --skip-grant-tables --skip-networking&

[root@MySQL ~]# mysqld_safe --user=mysql --skip-grant-tables --skip-networking&

　　运行之后新打开一个终端窗口：

[root@MySQL ~]# mysql -u root mysql

mysql> update mysql.user set authentication_string=password('mysql') where user = 'root';
Query OK, 1 row affected, 1 warning (0.02 sec)
Rows matched: 1  Changed: 1  Warnings: 1

mysql> flush privileges;
Query OK, 0 rows affected (0.00 sec)

mysql> exit
Bye

　　以上是5.7版本的修改密码方式，在5.7中存储密码的字段不再是password了，变成了authentication_string，在5.7之前采用以下命令：

mysql> UPDATE user SET Password=PASSWORD('newpassword') where USER='root';

　　修改之后登陆成功：

[root@MySQL ~]# mysql -uroot -p
Enter password: 
Welcome to the MySQL monitor.  Commands end with ; or g.
Your MySQL connection id is 6
Server version: 5.7.9-log MySQL Community Server (GPL)

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