Re: Idempotence and "Replication Insensitivity" are equivalent ?

From: paul c <toledobythesea_at_oohay.ac>
Date: Sat, 23 Sep 2006 01:37:24 GMT
Message-ID: <od0Rg.12923$1T2.10076@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 ...

p Received on Fri Sep 22 2006 - 20:37:24 CDT