It's often hard to ask relevant questions that will fit into the time allotted for an interview. Your DB schema example is a good one - it shows that the candidate can design (and to a lesser extent, implement) an application. Your example (I'm assuming) is missing strong algorithmic problem solving however. It would be good to have an additional question to test this kind of knowledge.

It's also important to realize that most technical interviews are really about weeding out bad candidates than finding good ones.

It may also be necessary to have questions general enough that any given candidate stands a chance at solving it. If a candidate doesn't have SQL experience your example would be bad (assuming the company didn't require SQL experience).

This is why you see questions like "implement a binary tree." Any competent programmer should be able to implement a binary tree. If they have never seen a binary tree before, they are simple enough to describe in 5 minutes.

I once asked a candidate to implement a trie. He had never heard of a trie before so I drew one on the whiteboard and briefly explained it. He asked me a few questions and then proceeded to implement one. We then discussed the pros and cons of different methods to store the nodes (vector, list, map). It showed me that, given a spec, he could code a data structure and the following discussion showed me he understood memory and time constraints.