Re: 4 the FAQ: Are Commercial DBMS Truly Relational?

"Christopher Browne" <cbbrowne_at_acm.org> wrote in message news:2srmk1F1njk29U1_at_uni-berlin.de...
>
> If you cut columns off of the result set, it is possible for the
> result set to, in fact, not be a "set", but rather a non-unique "bag"
> of tuples.
> [...]
> That is NOT going to be a "set" or a "relation" if some customer made
> multiple purchases between those dates.
>
> The problem that this expresses is that the relational algebra does
> not satisfy the property of closure.

That's an odd viewpoint.

The math books I've read have made the point that when you are talking about sets, then {2, 2} is the same set as {2}. So if you have duplicates after a project, you just throw them away; they "don't count" so to speak.

My understanding is that closure is a requirement to qualify as an "algebra."

> > Hrmn, uh, dunno. I guess I'm not sure I believe in the "relational
> > data model" as a fixed entity, although that notion is certainly
> > popular is some quarters. I myself am working on "a" relational
> > model that doesn't have NULLs of any kind, although it does have a
> > "systematic treatment of" optional data.
>
> I rather think that there need to be _some_ common properties...

Certainly.

Marshall Received on Sun Oct 10 2004 - 05:53:42 CEST