Re: Idempotence and "Replication Insensitivity" are equivalent ?

"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.

The problem arises when you project a bag of tuples into a domain. Now the projection is going to eliminate some duplicates, even if the projection includes ALL the original columns. If the bag of tuples was intended to model
a different reality than the set you get by projecting the bag onto all the columns, then you can expect different results at the end of the day. Hence the issue about "duplication sensitivity".

The reason relational operators don't exhibit "duplication sensitivity" is that relations don't contain duplicates. This is the part that isn't getting through, at all. Received on Fri Sep 22 2006 - 18:46:25 CEST