Re: Idempotence and "Replication Insensitivity" are equivalent ?

From: David Cressey <>
Date: Sat, 23 Sep 2006 08:17:39 GMT
Message-ID: <D46Rg.43$0Y2.0_at_trndny09>

"paul c" <> wrote in message news:od0Rg.12923$1T2.10076_at_pd7urf2no...
> David Cressey wrote:
> > "Chris Smith" <> wrote in message
> >
> >> <> wrote:
> >>> I have problems to follow you here. Has I said I know nothing about
> >>> theory. Do not know what you mean by the term "projection of
> >>> relations".
> >>> Is it something simple to grasp?
> >> It just means that you form a new relation which contains a subset of
> >> the information in the first relation by choosing some of the columns.
> >> If you have an n-ary relation of the form A1 x A2 x A3 x ... x An, then
> >> there are 2^n - 1 possible projections (excluding the project that
> >> selects no columns, because it's useless; but quite arbitrarily
> >> including the identity projection, which is just the original
> >> Because a relation is a set, the projection will combine any tuples
> >> have duplicate values in ALL of the projected columns. So if you have:
> >
> > Chris, your answer is correct and complete when it comes to projecting
> > relation into a domain.
> > ...


> David, it's probably me (apologies if I sound like I'm too much into the
> plonk once again), but maybe there's a slight chance it's you, so I'll
> ask. What does it mean to PROJECT a relation onto a domain? Is the
> relation referring in all ways, only to a domain? Or is such a relation
> secondary in some way? Or is this a different kind of lingo than what
> usually comes up here? I'm just guessing here ...

It's probably me. I'm trying to use the term "domain" the way some of the mathematicians seem to be using it.

Here's what I think I got by making inferences from other posts: the domain of a relation is the cartesian product of the domains of each of its attributes.

I probably misinterpreted something I read in here. That'll teach me to learn things in c.d.t.! Received on Sat Sep 23 2006 - 10:17:39 CEST

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