[dragonwriter]

> Is this really a SQL feature or a relational feature?

SQL.

There is no reason you couldn't have a composable implementaiton of relational logic.

> The essence of the relational model is that names are known up front

That every relation consist of tuples each member of which consists of a name, a type, and a value is part of the model, sure. That doesn't impose restrictions on composability.

> an attribute of one relation is not an attribute of another.

An attribute of one relation may well be an attribute of another.

There's no reason you couldn't have a relational language that let you store and reuse clauses.