pi@raspberrypi:~$ mysql -uroot -p
Enter password: [password given]
ERROR 1698 (28000): Access denied for user 'root'@'localhost'
1

pi@raspberrypi:~$ sudo !!
sudo mysql -uroot -p
Enter password: [same password as above]
Welcome to the MariaDB monitor.  Commands end with ; or g.
Your MariaDB connection id is 8
Server version: 10.1.38-MariaDB-0+deb9u1 Raspbian 9.0
[...]

sudo apt install mariadb-server mariadb-client

sudo mysql_secure_installation