MySQL问题:Access denied for user 'mysql'@'localhost'

deep@deep-PC:~$ sudo mysql -uroot -p

mysql> update mysql.user set authentication_string=PASSWORD('mysql'), plugin='mysql_native_password' where user='root';
Query OK,  row affected,  warning (0.04 sec)
Rows matched:   Changed:   Warnings: 

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

mysql> exit
Bye
deep@deep-PC:~$ sudo service mysql restart

　deep@deep-PC:~$ mysql -uroot -p