  To help a user specify and verify quantified queries --- a class of database
queries known to be very challenging for all but the most expert users --- one
can question the user on whether certain data objects are answers or
non-answers to her intended query. In this paper, we analyze the number of
questions needed to learn or verify qhorn queries, a special class of Boolean
quantified queries whose underlying form is conjunctions of quantified Horn
expressions. We provide optimal polynomial-question and polynomial-time
learning and verification algorithms for two subclasses of the class qhorn with
upper constant limits on a query's causal density.
