Re: Does Codd's view of a relational database differ from that ofDate&Darwin?[M.Gittens]
From: Jan Hidders <jan.hidders_at_REMOVETHIS.pandora.be>
Date: Tue, 28 Jun 2005 19:52:04 GMT
Message-ID: <EThwe.132522$eO.7105566_at_phobos.telenet-ops.be>
>>Let me mention two small things. In the RM if you want to add a
>>one-to-many relationship between two entities you have to extend one of
>>the relations with a foreign key. If there are more than one candidate
>>key you have to choose one of them. In an ER model you don't have to
>>make such a choice, you simply indicate that there is a relationship.
>>
>>Another small thing is updating primary keys. If a primary key has
>>accidentally been entered wrong and you want to fix that with an update
>>then it is usually not possible to simply update it, and the problem
>>gets even worse if it is also refered to by foreign keys. In an ER model
>>this is a non-problem.
Date: Tue, 28 Jun 2005 19:52:04 GMT
Message-ID: <EThwe.132522$eO.7105566_at_phobos.telenet-ops.be>
Jon Heggland wrote:
> In article <6O_ve.131771$Vs4.7201818_at_phobos.telenet-ops.be>, > jan.hidders_at_REMOVETHIS.pandora.be says... >
>>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?
?? You are asking how one indicates a relationship in the ER model?
> The ER model is not formal, and it's conceptual rather
> than logical.
I'm speaking loosely of ER-like models here, and for these there are
already several formalizations known. See for example the work on
ORM/NIAM. Formalizing the ER model is a no-brainer and that makes it a
data model that you can compare with the RM.
- Jan Hidders