Re: Does Codd's view of a relational database differ from that ofDate&Darwin?[M.Gittens]
Date: Tue, 28 Jun 2005 09:27:13 +0200
In article <6O_ve.131771$Vs4.7201818_at_phobos.telenet-ops.be>,
> Let me mention two small things. In the RM if you want to add a
> one-to-many relationship between two entities you have to extend one of
> the relations with a foreign key. If there are more than one candidate
> key you have to choose one of them. In an ER model you don't have to
> make such a choice, you simply indicate that there is a relationship.
> Another small thing is updating primary keys. If a primary key has
> accidentally been entered wrong and you want to fix that with an update
> then it is usually not possible to simply update it, and the problem
> gets even worse if it is also refered to by foreign keys. In an ER model
> this is a non-problem.
Isn't this just hand-waving? How exactly do you "indicate" the relationships? The ER model is not formal, and it's conceptual rather than logical. I don't see how it makes sense to compare them.
-- JonReceived on Tue Jun 28 2005 - 09:27:13 CEST