There are two cases depending on whether you installed mysql with the official binary installer or using macports:
MacPorts
sudo launchctl unload -w /Library/LaunchDaemons/org.macports.mysql.plist sudo launchctl load -w /Library/LaunchDaemons/org.macports.mysql.plist
Note: this is persistent after reboot.
Binary installer
sudo /Library/StartupItems/MySQLCOM/MySQLCOM stop sudo /Library/StartupItems/MySQLCOM/MySQLCOM start sudo /Library/StartupItems/MySQLCOM/MySQLCOM restart