I used XTDB on a side project (written in Clojure) a couple years ago and was blown away by something that I think isn't even really stressed as a headliner feature for XTDB. That was the ability to treat queries as first-class data. XTDB's datalog queries can be provided as just native Clojure data structures, and that allows you to build complex queries using code. What this means is that it's much more DRY than copy-pasting and editing SQL templates, there's no ORM gumming up your performance and expressivity as your queries get more complex, and there's no half-baked implementation of an API for query composition: the _programming language_ is your tool for query composition. I couldn't tell you if there are other ways of getting this, as I'm not really a software engineer anymore, but it was a jaw-dropping experience for me to see how easy it could be to write a database layer, and far and away the best experience I've ever had writing one in any programming language.

I suppose what's at the heart of this, more than anything, is that Datalog has a near-trivial syntax that can transparently be accommodated by two simple data structures: lists and hash tables. Not so with SQL, whose COBOL-y nature obscures its syntax and makes it less straightforward to represent in everyday data structures.