In my experience, PostgreSQL has the most helpful, supportive, and welcoming community of any software I've used. MySQL is basically a burning trash heap -- and so is the community around it. 😉
SQL Server isn't bad, but the local group seems a little cold to me, hard to break in, and I've had a really hard time finding good resources for the 70-462 exam beyond the official book, which I find incomplete compared to the practice exam.
The other community I've spent a lot of time in is the Perl community, which isn't bad overall, but does have a few bad apples / missing stairs.
I've stuck a toe in a few others, but nothing made me stick around.
I've had similar experiences with my local group. Sorta sad because I've been attended on and off since 1999.
Have you tried the MariaDB groups or do you just consider them part of the MySQL community?
I haven't used MariaDB, so I have no experience with that community. On the technical side, it's been a couple of years since I looked at it, but as far as I recall, it didn't seem significantly better than MySQL.