Re: One-To-One Relationships

From: David Cressey <cressey73_at_verizon.net>
Date: Thu, 29 Nov 2007 12:06:58 GMT
Message-ID: <CXx3j.20651$ht1.17107_at_trndny01>


"Bob Badour" <bbadour_at_pei.sympatico.ca> wrote in message news:474dd64a$0$5285$9a566e8b_at_news.aliant.net...
> rpost wrote:
>
> > paul c wrote:
> >
> > [...]
> >
> >>Regarding ER, here are some quotes from Codd's book (available for free
> >>at acm.org). The sarcasm of the second one made me laugh.
> >
> > The criticisms you quote may be amusing, and they have merit, but
> > they ultimately miss the point. The distinction between entities and
> > relationships: entities have identity (they can be referred to;
attributes
> > can have entity-valued domains), while relationships do not (they are
> > completely identified by their, possibly entity-valued, attributes).
>
> Except that relationships have identities too. The identity of the
> relationship between an employer and an employee, for example, is a
> contract. The relationship between a mother and a child, for example, is
> the event of birth. etc.
>
>
> > [...]
> >
> >>3. Even if this distinction had been precisely defined, it would have
added
> >>complexity without adding power. Whatever is conceived as entities, and
> >>whatever is conceived as relationships, are perceived and operated
> >>upon in the relational model in just one common way: as relations.
> >
> > This is the exact problem Chen identified. In the relational model
> > it is impossible to have entity-valued attributes, which, in practice,
> > we have a huge amount of.
>
> Entities are figments of our imaginations.
>
>
You are an entity. Received on Thu Nov 29 2007 - 13:06:58 CET

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