> An enum is a relation

This is TRUE. Most people ignore the relational model despite being so simple to learn. But one important thing to note, is that similar how you can say "anything is a object" you can say "anything is a relation".

This is valid SQL:

    SELECT * FROM (SELECT 1) a

And because "1" is a relation, a enum is.

Normal rdbms fail at operate to inner values (like the array feature on postgresql) and don't allow to put tables into cells. This is valid at the relational level but you need to do hacky stuff for common rdbms.

    SELECT Head =(SELECT * FROM INVOICE)

I'm working in a relational language, and this will be valid:

    for i in 1 do

and

    select value:Some(1).... where value.isSome