Interesting and powerful looking, but no releases or significant community activity since 2013? There are some answers in StackOverflow, so it doesn't seem to be dead by any means, but worrisome.
The issue tracker is unfortunately private, we fix bugs or implement minor features that materially affect RexDB paying customer's usage of HTSQL.
There is no such thing as "complete" in software development.
Any project wihout relatively recent commits or releases usually signifies abandonment and makes it worrisome to use, for me at least.
http://demo.htsql.org/course.filter(%0A%23%20include%20only%...