I’d like to chime in here, possibly a little late to the party. Someone mentioned SQL Server as a possibility. While I think SQL Server is actually a very good product, I think it’s probably not necessarily a good choice for a non-profit, volunteer-run effort because of licensing costs. I remember some of the discussions we had about licensing SQL Server when I was at SE, and the numbers were eye-popping. If we hope that Codidact scales (and it seems that everyone does hope that), then I’d say that would be an issue to avoid. It is much harder to switch databases later. PostgreSQL seems like the obvious choice to me.
@aggieben Welcome to Codidact. Don’t worry, we have already come to a pretty firm consensus that PostgreSQL is the way to go - well established, free, arguably more powerful than MySQL, etc.
The devil, of course, is in the details.