sudo apt remove --purge mysql-server
sudo apt purge mysql-server
sudo apt autoremove
sudo apt autoclean
sudo apt remove dbconfig-mysql

sudo service mysql stop #or mysqld
sudo killall -9 mysql
sudo killall -9 mysqld
sudo apt-get remove --purge mysql-server mysql-client mysql-common
sudo apt-get autoremove
sudo apt-get autoclean
sudo deluser mysql
sudo rm -rf /var/lib/mysql
sudo apt-get purge mysql-server-core-5.5
sudo apt-get purge mysql-client-core-5.5