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

From: Jon Heggland <>
Date: Mon, 13 Jun 2005 11:00:18 +0200
Message-ID: <>

In article <zmnqe.1639707$Xk.1222035_at_pd7tw3no>, says...
> if the question means something a little different, say "what are the
> salaries of the Personnel who might have sold the 'r' product?", would
> the answer (for whatever it's worth) not be the set of salaries of
> present personnal?

No, that information is not in the database either. You could join the relvars (resulting in their product, as you say), but the semantics of that relation would be just "There exists a product with price P and size Sz, and a person with age A and salary Slr." Not very useful, and it says nothing about selling. The information simply isn't there.

> my cut at all this is that "how old is the big $100 product" is an
> example of a kind of question that can't be answered by the RM, given
> this schema.

It can't be given by ANY data model, if the information is not there.

> the "what are the salaries ..." question might be
> pointless in practice. it does seem to be of the kind that can be
> answered but i'm not sure if this is always so.

The "kind" does not enter into it. If the information is there, the RM can produce it (it is complete). If it isn't, no model can.

> i suppose my question comes down to this: is it useful to limit RM use
> to questions of the second kind? ie. do we really need to answer any
> other questions besides ones that are patently answerable with a set of
> tuples? or to put it more simply, do we need any answers that aren't
> sets of tuples?

(Relations, you mean.) Not in the RM---it would complicate it, and violate the closure of the algebra. Of course, the ability to ascertain whether a given database can answer a given question is useful, but I don't think we need another model for that.

BTW, would you mind capitalising your sentences?

Received on Mon Jun 13 2005 - 11:00:18 CEST

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