There are different cases depending on whether you installed MySQL with the official binary installer, using MacPorts, or using Homebrew:
Homebrew
brew services start mysql
brew services stop mysql
brew services restart mysql
MacPorts
sudo port load mysql57-server
sudo port unload mysql57-server
Note: this is persistent after a reboot.
Binary installer
sudo /Library/StartupItems/MySQLCOM/MySQLCOM stop
sudo /Library/StartupItems/MySQLCOM/MySQLCOM start
sudo /Library/StartupItems/MySQLCOM/MySQLCOM restart