"I don't have a clue how one would be able to build a gui with it, or how one can easily interact with a database."
Because SQL is functional programming. Functional programming doesn't just mean Haskell, it also means Microsoft Excel (which, I've been told, is the most used programming platform in existence).
Excel is basically a wrapper around spaghetti code. It's great for small presentations, but using it for real programming is incredibly risky. There are an enormous number of stories floating out there about business apps written in Excel that turned out to have subtle but sometimes disastrous bugs due to random human errors as well as poor maintenance when the "program" changed hands.
reduce = GROUP BY
This is pretty much what I do at work; we have massive quantities of geographic and linguistic data that we have to translate to [REDACTED], and we do this by uploading it all into PostgreSQL and then running through a long sequence of "CREATE TABLE foo AS SELECT hairy_expression FROM bar;" statements.
For the time it was written (1970s, I believe) the SQL language was way ahead of the curve compared to the languages being used in enterprise.
I think I see your point, but is "natively nonfunctional" how you would describe lisp? There's not really any difference. One is binary relations and one is set relations.