To be frank, knowing the difference in how to write an inner and outer join when given a three table schema and a desired output is a frightening filter of candidates. Having an instinct that some type of index could help a query probably makes you a db wizard.
Even just asking for a basic understanding at the "use this to look up that" level -- since SQL syntax is easy to learn if you know what you want, and whoever does the recruiting has taken to mostly finding us new college graduates -- filters out an absurd number of candidates.
I guess it's the same thing as makes FizzBuzz a useful question.
Sure, I write a SQL query now and again, but if I have to join I look it up. I don't do it a lot. If I had to do it a lot for you, I would learn it inside out.
If I told you I was a DB wizard, then sure, I better already know how to join. But I know tons of people that don't really do SQL. They are eminently hire-able and valuable.
source: a guy (me) who was bounced out of an interview because he didn't recall some specific detail about boost::shared_pointer().
I think at this point, most HN'ers would accept that a work sample test (of some limited scope and effort) is a better test than any interview question or whiteboard coding process. From that perspective, no question is really a good one. Unfortunately, at my workplace, there are strict rules in place specifically prohibiting this type of applicant process.