If you don't know how to do a JOIN you really don't know how to use SQL, and that's not a tricky use of JOINs at all. If you're hiring for a position that mostly uses an ORM or something and raw SQL is only important in cases where the ORM is giving bad performance, maybe not knowing how to do a three-table join is fine. If actually writing SQL is part of the job regularly, though, the sort of thing grandparent is talking about is absolutely a bare-minimum of knowledge needed, maybe even below the bare minimum.

To clarify, two SQL questions were asked during an interview round for a job description that specifically required the ability to write ANSI SQL against databases for reporting/analytics purposes. Opposed to bringing candidates in for a job description requiring Java experience and then dropping a couple of SQL "gotcha" questions on a surprised candidate.

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.