Comment by int_19h

I'm kinda surprised that the list of influences doesn't mention XQuery. Yes, it's not a relational query language... but it covers much of the same ground in practice, especially the part that they call "FLWOR expressions" (for/let/where/order/return) that operate on "tuple streams":

https://www.w3.org/TR/xquery-31/#id-flwor-expressions

And it has grouping, windowing functions etc. I bet you could define a subset that is specifically tailored to the same use cases as SQL - basically, get rid of everything to do with elements and attributes, and only allow scalars and sequences (and maybe maps?). But otherwise keep XDM data types and their semantics.