On Thu, Sep 22, 2016 at 04:02:37PM +0200, Tony van der Hoff wrote:
> >>>> Running Jessie here. Performed apt-get upgrade yesterday, which included
> >>>> a new version of mysql.
> It turns out that the recent upgrade took mysql from 5.5 to 5.6. It
> uninstalled mysql-server-5.5, but failed to install mysql-server-5.6,
> due to an unmet dependency on mysql-server-core-5.6.
Sounds like you're getting mysql-server from jessie-backports instead
of jessie. On a pure jessie system, it is still 5.5.