drop and install mysql on ubuntu

//drop my sql on ubuntu
https://askubuntu.com/questions/172514/how-do-i-uninstall-mysql
https://www.digitalocean.com/community/questions/mysql-installation-error-dpkg-error-processing-package-mysql-server-5-5-configure


// upgrade ubuntu
https://askubuntu.com/questions/830783/how-to-upgrade-my-ubuntu-15-04-to-ubuntu-16-04

// install my sql from instruction
https://dev.mysql.com/doc/refman/5.7/en/

// change default password
https://superuser.com/questions/603026/mysql-how-to-fix-access-denied-for-user-rootlocalhost