Re: One-To-One Relationships

From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Wed, 28 Nov 2007 16:57:44 -0400
Message-ID: <474dd64a$0$5285$9a566e8b@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.

   When I say a particular person is the head
> of a particular department, I do not mean to assert a relationship
> between particular combinations of attribute values for departments
> and persons; I may not even know what these attributes are.

You are confusing yourself with nebulous terminology. (Exactly as Codd's quotes would predict.) A relation is a set of truth statements. When I say a particular person is the head of a particular department, I make exactly such a truth statement.

[further nonsense snipped unread] Received on Wed Nov 28 2007 - 14:57:44 CST