Something like the DOT language used in GraphViz

https://graphviz.org/doc/info/lang.html

Yes DOT (and the other UML whatever languages) are absolutely the only extant examples that make an attempt. But again, if you look at DOT you'll see it doesn't actually do anything syntactically - it just has syntax for edge lists.

Any syntax with a let operator to name stuff or a lambda abstraction.

That only gives you trees not DAGs - you can't do fan-in (there's no way to "share" let bound names).

Isn't this fan-in?

let A = select * from tbla

let B = select * from tblb

let C = select * from A join B

Think about what tbla and tblb can be - hint they can only be globals (ie actual tables)

A = select * from tbla

B = select * from tblb

C = select * from A join B

I guess CTEs already provide that (even if they're a bit clunky).

    WITH a AS (select * from tbla),
    b AS (select * from tblb)
    select * from a join b

Yes we already have that, it's called `async def`.