Right. Mariadb is a drop-in replacement for MySQL by the original MySQL author, and is installed by default since a few years (I think since 13.1, might have been 12.3 already though). Both provide a “mysql-server”.
In a default KDE installation, at least Amarok, Akonadi, and digikam need some "mysql-server, i.e. mysql-community-server or mariadb, to be installed. (though digikam uses sqlite by default)
You can of course lock mariadb, then a “zypper remove mysql-community-server” will offer you to uninstall everything that requires mysql/mariadb.