Re: Idempotence and "Replication Insensitivity" are equivalent ?
Date: Sat, 23 Sep 2006 01:37:24 GMT
Message-ID: <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. > ...
p Received on Sat Sep 23 2006 - 03:37:24 CEST