Hi,
I think I must have accidentally deleted a folder from my database some time ago. How can I restore a folder from my Time Machine backup, and not the entire database?
You can’t. They don’t exist in the filesystem, so you’d need to restore the entire database.
I had a similar problem and I got DB from Time Machine, left in one folder different of the DT one, then opened that DB with DT, and moved the deleted items to my normal DB. Then closed the mounted DB and deleted it.
Said that, DT has its internal recycler bin, when you empty it, it goes to the macOS recycler, and even that we, as humans, with those two security steps, we still make mistakes. 
My philosophical Sunday Morning (in Europe) two cents. 
Yes,you can also restore to a different location, open both, and cherry pick items, if the situation requires it. However, make sure to close and delete the old one when you’re done.