Comment 11 for bug 1592749

I suspect the problem is that purging MariaDB doesn't remove the database (the maintainer scripts are supposed to ask, but seems there's a bug with this). MariaDB and MySQL are no long valid drop-in replacements of each other, so MySQL won't be able to to work with a MariaDB database, which is probably why the installation fails. rm -rf -ing /var/lib/mysql and /etc/mysql should fix it.

I can try testing with MariaDB on my box and see if I get the same result, at least.