What other approaches to making SQL composable can we imagine? With functions it seems very simple to extract a WHERE clause into a predicate of some sort. I'm sure even if this exact syntax isn't preferable, being able to reuse join logic would be great.

That said, in the case of wanting to abstract out or reuse joins, just write a view, I guess. And I get a lot of mileage in Postgres from just writing functions to abstract out predicates, because it allows you to write things like `select * from order where order.is_complete` instead of `where is_complete(order)`