MySQL問題: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, 1 row affected, 1 warning (0.04 sec)
Rows matched: 1  Changed: 1  Warnings: 1

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

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

　deep@deep-PC:~$ mysql -uroot -p