如果在 MySQL 5.7.+ 中有这个问题：

Access denied for user 'root'@'localhost'

因为 MySQL 5.7 默认允许通过 socket 连接，这意味着您只需使用管理员权限连接到 MySQL。

SELECT user,authentication_string,plugin,host FROM mysql.user;

t然后就看到

+------------------+-------------------------------------------+-----------------------+-----------+
| user             | authentication_string                     | plugin                | host      |
+------------------+-------------------------------------------+-----------------------+-----------+
| root             |                                           | auth_socket           | localhost |
| mysql.session    | *THISISNOTAVALIDPASSWORDTHATCANBEUSEDHERE | mysql_native_password | localhost |
| mysql.sys        | *THISISNOTAVALIDPASSWORDTHATCANBEUSEDHERE | mysql_native_password | localhost |
| debian-sys-maint | *497C3D7B50479A812B89CD12EC3EDA6C0CB686F0 | mysql_native_password | localhost |
+------------------+-------------------------------------------+-----------------------+-----------+
4 rows in set (0.00 sec)

允许使用管理员权限连接并输入密码，然后通过命令更新表中的值：

ALTER USER 'root'@'localhost' IDENTIFIED WITH mysql_native_password BY 'Current-Root-Password';
FLUSH PRIVILEGES;

然后再次运行select命令，你就会看到它已经更改了：

+------------------+-------------------------------------------+-----------------------+-----------+
| user             | authentication_string                     | plugin                | host      |
+------------------+-------------------------------------------+-----------------------+-----------+
| root             | *2F2377C1BC54BE827DC8A4EE051CBD57490FB8C6 | mysql_native_password | localhost |
| mysql.session    | *THISISNOTAVALIDPASSWORDTHATCANBEUSEDHERE | mysql_native_password | localhost |
| mysql.sys        | *THISISNOTAVALIDPASSWORDTHATCANBEUSEDHERE | mysql_native_password | localhost |
| debian-sys-maint | *497C3D7B50479A812B89CD12EC3EDA6C0CB686F0 | mysql_native_password | localhost |
+------------------+-------------------------------------------+-----------------------+-----------+
4 rows in set (0.00 sec)

然后这就是它。在完成sudo mysql_secure_installation命令之后，您可以运行以下过程。

对于mariadb，使用：

SET PASSWORD FOR 'root'@'localhost' = PASSWORD('manager');

然后就是设置密码。更多可以看https://mariadb.com/kb/en/set-password/。

使用 resetting 密码的说明 - 但不是重置密码，而是将一个记录强制插入到 mysql.user 表中

INSERT INTO mysql.user (Host, User, Password) VALUES ('%', 'root', password('YOURPASSWORD'));
GRANT ALL ON *.* TO 'root'@'%' WITH GRANT OPTION;