Debian 9, the default MySQL variant is now MariaDB

https://www.debian.org/News/2017/20170617 에서 일부 발췌

Debian 9 Stretch released
June 17th, 2017

In Stretch, the default MySQL variant is now MariaDB. The replacement of packages for MySQL 5.5 or 5.6 by the MariaDB 10.1 variant will happen automatically upon upgrade.

Debian 9.5 Released.
https://www.debian.org/News/2018/20180714