#sudo apt-get -y install mysql-server mysql-client
// 登录MySQL，MySQL的root用户密码默认空
#sudo mysql -u root
 

Basically means that: db_users using it, will be "auth" by the system user credentias. You can see if your root user is set up like this by doing the following:
 
$ sudo mysql -u root # I had to use "sudo" since is new installation
 
mysql> USE mysql;
mysql> SELECT User, Host, plugin FROM mysql.user;
 
+------------------+-----------------------+
| User             | plugin                |
+------------------+-----------------------+
| root             | auth_socket           |
| mysql.sys        | mysql_native_password |
| debian-sys-maint | mysql_native_password |
+------------------+-----------------------+
As you can see in the query, the root user is using the auth_socket plugin
 
There are 2 ways to solve this:
 
You can set the root user to use the mysql_native_password plugin
You can create a new db_user with you system_user (recommended)
Option 1:
 
$ sudo mysql -u root # I had to use "sudo" since is new installation
 
mysql> USE mysql;
mysql> UPDATE user SET plugin='mysql_native_password' WHERE User='root';
mysql> FLUSH PRIVILEGES;
mysql> exit;
 
$ service mysql restart
Option 2: (replace YOUR_SYSTEM_USER with the username you have)
 
$ sudo mysql -u root # I had to use "sudo" since is new installation
 
mysql> USE mysql;
mysql> CREATE USER 'YOUR_SYSTEM_USER'@'localhost' IDENTIFIED BY '';
mysql> GRANT ALL PRIVILEGES ON *.* TO 'YOUR_SYSTEM_USER'@'localhost';
mysql> UPDATE user SET plugin='auth_socket' WHERE User='YOUR_SYSTEM_USER';
mysql> FLUSH PRIVILEGES;
mysql> exit;
 
$ service mysql restart
Remember that if you use option #2 you'll have to connect to mysql as your system username (mysql -u YOUR_SYSTEM_USER)
 
Note: On some systems (e.g., Debian stretch) 'auth_socket' plugin is called 'unix_socket', so the corresponding SQL command should be: UPDATE user SET plugin='unix_socket' WHERE User='YOUR_SYSTEM_USER';