Re: Idempotence and "Replication Insensitivity" are equivalent ?
Date: Sat, 23 Sep 2006 08:17:39 GMT
Message-ID: <D46Rg.43$0Y2.0_at_trndny09>
"paul c" <toledobythesea_at_oohay.ac> wrote in message
news:od0Rg.12923$1T2.10076_at_pd7urf2no...
> David Cressey wrote:
> > "Chris Smith" <cdsmith_at_twu.net> wrote in message
> > news:MPG.1f7ca1de6d803d8598972e_at_news.altopia.net...
> >> <pamelafluente_at_libero.it> 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
relation).
> >> Because a relation is a set, the projection will combine any tuples
that
> >> 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
a
> > 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