Good question. We don't have a DBA either. I've learned SQL as needed and while I'm not terrible, it's still daunting when making the schema for a new module that might require 10-20 tables or more.

One thing that has worked well for us is to alway include the top-most parent key in all child tables down yhe hierarchy. This way we can load all the data for say an order without joins/exists.

Oh and never use natural keys. Each time I thought finally I had a good use-case, it has bitten me in some way.

Apart from that we just try to think about the required data access and the queries needed. Main thing is that all queries should go against indexes in our case, so we make sure the schema supports that easily. Requires some educated guesses at times but mostly it's predictable IME.

Anyway would love to see a proper resource. We've made some mistakes but I'm sure there's more to learn.