Using package management is great. Adding MySQL to your box is a doddle! Unfortunately, getting rid of MySQL once you’ve installed it leaves stuff behind.
You will find that your databases and user configurations (including usernames and passwords) are left on your box even after you have used your package manager to remove MySQL.
So, use the following to get rid of it totally:
apt-get purge mysql-server mysql-common mysql-client-<version> apt-get autoremove rm -rf /var/lib/mysql rm -rf /etc/mysql*