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Home -> Community -> Usenet -> comp.databases.theory -> Re: Does Codd's view of a relational database differ from that ofDate&Darwin?[M.Gittens]

Re: Does Codd's view of a relational database differ from that ofDate&Darwin?[M.Gittens]

From: Jon Heggland <heggland_at_idi.ntnu.no>
Date: Fri, 1 Jul 2005 10:16:34 +0200
Message-ID: <MPG.1d2ccb1d3ee567b69896bf@news.ntnu.no>


In article <THxwe.14760$eM6.3208_at_newsread3.news.atl.earthlink.net>, david.cressey_at_earthlink.net says...
> 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.

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.

-- 
Jon
Received on Fri Jul 01 2005 - 03:16:34 CDT

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