Regardless of whether we're interviewing for a DBA or a developer who will code SQL, one question I ask is:
"For the next 15 minutes, explain everything you know about locking, blocking, and isolation level."
Yes, that's a broad topic, but I can tell a LOT about someone's past experience and areas of expertise based on that one question.
"Do not seek to follow in the footsteps of the wise. Instead, seek what they sought." - Matsuo Basho