mysql -u root doesnt work but sudo mysql -u root does, why?

Solution 1:

Access with sudo: sudo mysql -u root -p
Delete the root user: drop user 'root'@'localhost';
Create the root user again: create user 'root'@'%' identified by 'your_password';
Give permissions: grant all privileges on *.* to 'root'@'%' with grant option;
Update permission tables: flush privileges;
Exit MYSQL and try to reconnect without sudo.

Font: can't login as mysql user root from normal user account in ubuntu 16.04