1. DO NOT forget to backup your data.
2. Stop the database server.
To uninstall MySQL and completely remove it (including all databases) from your Mac, do the following from the Terminal window:
rm -rf ~/Library/PreferencePanes/My*
sudo rm /usr/local/mysql
sudo rm -rf /usr/local/mysql*
sudo rm -rf /Library/StartupItems/MySQLCOM
sudo rm -rf /Library/PreferencePanes/My*