Thanks very much for forwarding your suggestion. Whilst I think that there would definitely be market for it, unfortunately, MS SQL Server Express is not open source software.

Whilst I understand that MS SQL Server Express is "free" (in the sense of no monetary cost - i.e. freeware), it's not "free" in the sense of "free software" as it has a proprietary license.

Due to our commitment to open source software, plus the fact that redistributing proprietary software can be a legal minefield (although I do note that SQL Server Express does appear to be ok to redistribute), we only package open source software for now. Perhaps MS will release it under a free/open source license at some point?

I say, never say never; so perhaps one day we'll reconsider this idea. But not at the moment, and in honesty, I don't anticipate that our policy will change any time soon.

Having said that, there is no reason why you couldn't install MS SQL Server Express on top of Core yourself (or on one of the other servers if you think it'd make a better base). TurnKey servers are based on Debian (v15.x = Debian 9/Stretch) so most instructions for Debian, will also work on TurnKey. I note that Microsoft don't provide instructions for installing SQL Server on Debian, but Ubuntu is also based on Debian, so I suspect that with a few adjustments, the Ubuntu instructions would also work on TurnKey.

If you're interested in going that path, please let me know. I have a few things on ATM, but when I get a chance, I'd be happy to convert those docs to something that would allow you to easily install SQL Server on TurnKey using "best practice". Alternatively, if you're in a rush, have a go at following their instructions and post back with any issues you hit and I'll help you out.