delete mysql data file
sudo rm /var/lib/mysql/ -R
delete mysql configuration file
sudo rm /etc/mysql/ -R
Automatically uninstall mysql (including server and client)
sudo apt-get autoremove mysql* --purge sudo apt-get remove apparmor
Check if uninstall is clean
dpkg -l | grep mysql # 若没有返回，说明已完成卸载
The installation is then a simple matter
sudo apt-get install mysql-server mysql-client
In this way, the version of mysql 5.5 is installed by default, and then try to install mysql 5.7, and add the installation of 5.7 later.
forward from:
https://blog.csdn.net/qq_25730711/article/details/53503959
The test passed on ubuntu16.04