It's my understanding that zero-knowledge concepts can be extended to databases, in a way where I can query a database and get a result without knowing the contents, perhaps to pass on to a trusted system to perform another action.

Depending how you slice it, I either do or don't get to know if the result set is empty. In the latter case, there's a variation of this crush registration system where I don't get to know if Sally has a crush on me, but I do find out if anyone has a crush on me. Of course if the answer is empty set, I don't know if I'm unloved or only loved by people that don't use the app. Which could be pretty heavy.