echo 'manual' | sudo tee /etc/init/mysql.override
# command from root shell
echo manual >> /etc/init/mysql.override
https://askubuntu.com/a/20347/861079
The configuration file /etc/init/SERVICENAME.conf contains a line indicating when to start the service: start on …. An easy way of disabling these services is to change that line to start on never and (…). If you don't want to edit the file, you can also completely disable the service without confusing the packaging system by renaming it to not end in .conf.
https://superuser.com/a/204062/765894