首先出现的情况是mysql -uroot -p123456可以登录，

但是网站配置了账号却显示Mysql:ERROR 1698 (28000): Access denied for user 'root'@'localhost'。

那就直接登录进去执行下面一条语句即可。 

mysql.user表里authentication_string这个字段是密码，以后会逐渐废弃password字段

UPDATE mysql.user SET authentication_string=PASSWORD('root的密码'), PLUGIN='mysql_native_password' WHERE USER='root';

如果命令行都登不上去的话，

使用cat命令查看默认用户名密码。

sudo cat /etc/mysql/debian.cnf

root:~# sudo cat /etc/mysql/debian.cnf 

# Automatically generated for Debian scripts. DO NOT TOUCH!

[client]

host     = localhost

user     = debian-sys-maint

password = 0XRu7iGdhb6UT8Af

socket   = /var/run/mysqld/mysqld.sock

[mysql_upgrade]

host     = localhost

user     = debian-sys-maint

password = 0XRu7iGdhb6UT8Af

socket   = /var/run/mysqld/mysqld.sock

使用默认用户名密码登录

root@ubuntu:~# mysql -udebian-sys-maint -p0XRu7iGdhb6UT8Afmysql: [Warning] Using a password on the command line interface can be insecure.Welcome to the MySQL monitor.  Commands end with ; or \g.Your MySQL connection id is 63Server version: 5.7.25-1 (Ubuntu)Copyright (c) 2000, 2019, Oracle and/or its affiliates. All rights reserved.Oracle is a registered trademark of Oracle Corporation and/or itsaffiliates. Other names may be trademarks of their respectiveowners.Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.mysql>

执行刚才那个更新语句，

退出来

重启mysql。

生效。

嗯。

实在解决不了，就卸载重装吧。

root@ubuntu:~#rm /var/lib/mysql/ -Rroot@ubuntu:~#rm /etc/mysql/ -Rroot@ubuntu:~#apt-get autoremove mysql* --purgeroot@ubuntu:~#apt-get remove apparmorroot@ubuntu:~#apt-get updateroot@ubuntu:~#apt-get install mysql-server