Re: RA with MV attributes

On Jan 17, 5:32 pm, "David" <davi..._at_iinet.net.au> wrote:
> Note that thinking of a type this way reminds us that it's
> nonsensical to say that the set of attributes of a relation (directly)
> represents its type. The type "relation" has more to do with the
> set of all possible sets of tuples.

If we know the set of attributes and their types, do we not also know the set of all possible sets of tuples? What's the difference?

> Marshall is correct in saying that the type of a relation can (if we
> desire) be parameterized on the attributes, and that this may indeed by
> useful sometimes. However, I don't think it's so useful in the
> mathematical definition of a relation. For example a join would no
> longer be regarded as a binary operation.
>
> From Wikipedia : "... a binary operation on a set S is a binary
> function from S and S to S"

Natural join is a binary operation closed over the set of all relations.

Marshall Received on Thu Jan 18 2007 - 06:47:04 CET