There is nothing wrong with preparing yourself for an interview (you should be), but in my opinion, not knowing all the answers is not that bad.

If you can show the interviewer that you have good knowledge of the product and that you know how to research "the gaps of your knowledge", you should be fine.

It is impossible to know everything about SQL Server (except for a few exceptional people), so there is no shame in not knowing an answer (however, it is a shame not knowing any answer :-)).

If you don't know the answer, write the question down and send an e-mail later to the interviewer with the correct answer and how you researched the problem. I know some people who got the job after this technique.