# 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_at_pd7urf2no>

>> <pamelafluente_at_libero.it> wrote:

>> the information in the first relation by choosing some of the columns.

Date: Sat, 23 Sep 2006 01:37:24 GMT

Message-ID: <od0Rg.12923$1T2.10076_at_pd7urf2no>

> "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. > ...

p Received on Sat Sep 23 2006 - 03:37:24 CEST