I wonder if that could be part of the assessment, to tell if the candidate is likely to verify uncertainties or make assumptions.

Now that you mention it, probably so.

I mean total revenue, but I leave it unspecified in the hope that someone will ask for clarification. Same with if they should include authors with no sales during the time period.

I like this question because the base query, however you interpret the unclear parts, is easy, but it still allows candidates to impress me by asking about the "edge" cases. And even if they just makes assumptions, I can still ask them to do the other versions to see if they really know what they're talking about. I'm always amazed at how many people can't convert an inner join to a left join, or can't change a sum() to a count().