Re: "Shouldn't an SQL engine not care about if you used IN, EXISTS, or JOIN if it is truly declarative shouldn't it just give you the correct answer in reasonable time if possible by any of the three methods?"
I don't see this as evidence SQL is not declarative. Declarative doesn't mean there must be only one way to do (request) something, and it doesn't mean the optimizer is required to be well-written in practice. Maybe in some RDBMS it does process all 3 fast. That's not a characteristic of SQL the language.
