Hi, there are a few more bits forgotten in the mysql-5.5 -> mysql-5.6 transition. E.g. digikam has Build-Depends: mysql-server-core-5.5 | mysql-server-core which is not satisfiable in stretch (any only by chance still works in sid). Luckily this seems to be the only package in stretch affected by this. I didn't check sid, there seem to be several false positives due to old source packages being still available in sid (for Built-Using or outdated binaries on some platforms).
No bug filed so far against digikam, maybe Otto can do this. For simplifying potential transitions between mysql and mariadb I'd suggest to introduce a set of default-mysql-* (or mysql-default-* or whatever you like) packages, probably comparable to mpi-default-{dev,bin}. IMO, these should be real packages that depend on the corresponding package from the (currently) preferred implementation. Unique virtual packages could work, too, but might make things more complicated. The virtual-mysql-* packages are not unique (there are multiple providers), so they cannot be used as the first alternative (or only) dependency. All packages that are not specific to one of the two implementations (i.e. can work with both alternatives equally), should have such a *-default-* package as the first alternative in their (Build-)Depends. If the packages have to be touched anyway to fix versions or add (mariadb) alternatives, maybe such an approach with *-default-* could be considered as a more generic solution. Andreas