Set MySQL 5.7.x Start at Boot on macOS
The Tip Show How to Getting MySQL Database Server Started at Boot Time on macOS.
The Startup Item for MySQL is installed into ‘/Library/StartupItems/MySQLCOM’.
If you want to disable the automatic startup of MySQL, simply change ‘MYSQLCOM=-NO-‘ on /etc/hostconfig.

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Getting-Started with MySQL 5.7+ Server on macOS
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Double-Click to Start MySQL StartUp Items Installer.
Solve App from Unidentified Developer
Here How Open Unsigned Mac AppTroubleshooting Mac Mavericks App can't be Opened because it is from and unidentified developer -
Strongly Recommended MySQL Server Database Secure SetUp
Here MySQL Secure Database SetUpLink to Guide on Post-Installation Secure SetUp of MySQL on Linux/Unix