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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: Jan Hidders <jan.hidders_at_REMOVETHIS.pandora.be>
Date: Fri, 01 Jul 2005 20:43:20 GMT
Message-ID: <IVhxe.135039$l56.6861917@phobos.telenet-ops.be>


Jon Heggland wrote:
> In article <dSCwe.133254$g25.7165020_at_phobos.telenet-ops.be>,
> jan.hidders_at_REMOVETHIS.pandora.be says...

>>
>>Who needs rows? We only have objects (or identities) and binary 
>>relationship between objects and values (attributes, like between a user 
>>and an e-mail address) and between objects and objects (relationships, 
>>like between a person and person-details). [...] 

>
> So a person has some "identity" that is independent of the values of
> his/her properties/details. It is the same idea as row ids, even if no
> "rows" as such are involved.

Yes, correct.

>>>>Yes, that is at the core of my suggestion: a decent theoretical
>>>>foundation for opaque keys, abstract identifiers, or whatever you want
>>>>to call them.

>
> A surrogate key (that you don't show anyone) fulfills the same purpose.

In some sense, but it would be a bit like saying that references fulfill the purpose of pointers you don't do pointer-arithmetic with. That's true, but it misses the essence. Opaque keys would come with a formalization that better captures what they mean and how they should be treated. It's the difference between informally stating that certain fields are surrogate keys and waving your hands a bit, and coming up with a precise formal theory of what that exactly means.

> [...] Perhaps I am skeptical to
> your proposals because from my point of view, identity separate from
> value makes thing more complicated, not simpler. And/or because looking
> at everything as objects/entities and relationships is a too restrictive
> worldview for me. I guess I would be happier if this was presented as a
> layer on top of the RM instead of as a replacement.

Well, if that makes you happier then by all means let us present it as such. :-) So where do you get the feeling hat this worldview would be somehow restrictive? What do you think that becomes more difficult to model?

> Anyway, there is one thing that is bugging me when I read articles about
> object data models like Calvanese/Lenzerini and Van den Bussche /
> Paredaens. If objects are just identity points, and the properties of
> objects are other objects, where do actual data values enter into the
> picture?

Data values are special objects that have one or more representations associated with them by which they are identified. Like LOTs and NOLOTs in NIAM.

Received on Fri Jul 01 2005 - 15:43:20 CDT

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