Can't login with Mysql root account but can login when use sudo command in ubuntu 16.04

Question

Can't login with Mysql root account but can login when use sudo command in ubuntu 16.04

bluebird:~$ mysql -u root -p
Enter password: 
ERROR 1698 (28000): Access denied for user 'root'@'localhost'
bluebird:~$ mysql -u root -p
Enter password: 
ERROR 1698 (28000): Access denied for user 'root'@'localhost'
bluebird:~$ sudo mysql -uroot
[sudo] password for bluebird: 
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 21
Server version: 5.7.13-0ubuntu0.16.04.2 (Ubuntu)

Copyright (c) 2000, 2016, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> 

When I use mysql -u root -p , I can't login. When I use sudo mysql -u root , i can.

And I don't have the password for the root user of MySQL.

I don't know why.

Answer 1

I used the command below:

use mysql;
select User, Host from user;

It turns out, somehow I got 'root'@'XXXXX-aspire','root'@'localhost' and 'root'@'%'.

When I delete 'root'@'XXXXX-aspire', I can login by mysql -uroot without problem.

Answer 2

Try this as SUDO then try login without sudo

GRANT ALL PRIVILEGES ON *.*  TO 'root'@'localhost';
FLUSH PRIVILEGES;