Re: Does Codd's view of a relational database differ from that ofDate&Darwin?[M.Gittens]
Date: Fri, 1 Jul 2005 10:16:34 +0200
Message-ID: <MPG.1d2ccb1d3ee567b69896bf_at_news.ntnu.no>
In article <THxwe.14760$eM6.3208_at_newsread3.news.atl.earthlink.net>,
david.cressey_at_earthlink.net says...
Yes, exactly. Which means comparing ER model(s) and the RM make little
sense. Conceptual vs. logical.
> "Foreign keys", implement relationships.
I disagree a little with this, though I realise that is what most people
think. I would rather say that common domains implements relationships.
Foreign keys *constrain* them.
> Here's some phrasing that I picked up about 20 years ago, when I was
> learning ER and RDM. I still like it:
>
> "In the ER model, relationships are identified but not implemented."
>
> The idea here is that lines on paper (or elements in a model) specify
> relationships, give them names, connect them with entities, and describe
> them. But they don't provide a specific data structure for "realizing"
> those relationships at retrieval time.
--
Jon
Received on Fri Jul 01 2005 - 10:16:34 CEST