Re: One-To-One Relationships
Date: Wed, 28 Nov 2007 16:57:44 -0400
> 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.
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 - 21:57:44 CET