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

From: Jon Heggland <>
Date: Fri, 17 Jun 2005 13:56:31 +0200
Message-ID: <>

In article <>, says...
> > Not at all. It knows constraints (of which foreign keys are an important
> > special case), it knows domains. It can suggest "join paths", but if
> > multiple paths are possible, some path (or combination of paths) must be
> > selected! This can be done in multiple ways; *how* is not an issue of
> > data model.
> That's approximately what I wrote to AS awhile ago. He appears to be
> claiming that RM 'explicit joins' are the 'instructions' one has to specify
> in order to retrieve data and that the 'instructions', a.k.a 'explicit
> joins', are eliminated in his model.

I understand him as saying that some "join paths" should be considered special, and others forbidden, thus simplifying the formulation of some queries (by not requiring the joins to be stated), but disallowing others altogether. In other words, the functionality that views provide, but with additional limitations.

A more generous reading might be that the other queries are not disallowed, but that their joins must be stated explicitly.

> Now, a more interesting question is what exacly AS's model is. It appears
> that it's different from concept lattices and I utterly failed to get a
> concise explanation from the author. Would you be able to explain the
> difference, by any chance ?

Sorry, but I am not familiar with concept lattices.

Received on Fri Jun 17 2005 - 13:56:31 CEST

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