Does the unit of composition have to be a complete query/CTE? Theoretically it seems like it could be any fragment of SQL, but the docs seem to imply it must be a complete query.

I created something similar recently: https://docs.racket-lang.org/plisqin/index.html. At its core, it is also a library for composing fragments of SQL, but it has some novel (as far as I know) ideas. The most notable is that joins are values (or "expressions", if you prefer) that can be returned from a procedure like any other value. I was hoping that the world would realize "that's obviously how query builders should work" and copy the approach when starting new projects, but that hasn't happened.