To uninstall MySQL and completely remove it (including all databases) from your Mac do the following:

• Open a terminal window
• Use mysqldump to backup your databases to text files!
• Stop the database server
• sudo rm /usr/local/mysql
• sudo rm -rf /usr/local/mysql*
• sudo rm -rf /Library/StartupItems/MySQLCOM
• sudo rm -rf /Library/PreferencePanes/My*
• edit /etc/hostconfig and remove the line MYSQLCOM=-YES-
• rm -rf ~/Library/PreferencePanes/My*
• sudo rm -rf /Library/Receipts/mysql*
• sudo rm -rf /Library/Receipts/MySQL*
• sudo rm -rf /private/var/db/receipts/*mysql*

The last three lines are particularly important as otherwise, you can’t install an older version of MySQL even though you think that you’ve completely deleted the newer version!

info from https://community.jaspersoft.com/wiki/uninstall-mysql-mac-os-x