Re: WWW/Internet 2009: 2nd CFP until 21 September

From: paul c <>
Date: Sat, 15 Aug 2009 18:12:12 GMT
Message-ID: <0UChm.41410$PH1.8663_at_edtnps82>

Mr. Scott wrote:
> ...
> What constitutes an "eligible" wff? ,,,

Obviously the wff's that are representable in one of Codd's relations. This is not the set of all wff's.

Remember that Codd endorsed the use of
> null in the relational model. He devoted two complete chapters to them in
> his book 'The Relational Model for Database Management, Version 2,'
> contrasting the four-valued logic of RMv2 to the three-valued logic needed
> in RMv1.

Codd had at least two models, the 1970 one is the simplest. He should have written some chapters on the logic of expressing two different relations in one representation/table. Eg., how this affects projection. Walter gave a clear example of an ORDERS table that is an attempt to represent two different relations, eg., one that has attributes {OrderId, CustomerId} and one that might have attributes {OrderId, CustomerId, OrderDate}.

... Isn't it true that in first order logic which is the basis for the
> relational model, Pabcdxy is an atomic well-formed formula even though x and
> y are variables? Doesn't Pabcdxy being true convey information about a, b,
> c and d, regardless of the values for x and y?

This is making the same mistake as saying that a French translation can express any English poem, on the grounds that English has borrowed many terms from French. Codd borrowed much from logic but he didn't reproduce all of it. He applied it in a specialized way that gets leverage from the repetitive features of typical computers and also to get results that logic can't get, eg., any language that supports 'insert' requires one to assert that 'fact a OR fact b' is equal to 'fact a AND fact b'. Here, 'fact a' and 'fact b' would correspond to tuples, but note that Codd never offered any relational operators for joining tuples, only relations. It is not a question whether relational logic reproduces classical logic but how classical logic can be applied to organize, manipulate and validate relations.

I am starting to think Bob B had it pegged, either you're not really serious about understanding the essence of Codd's theory and are relying on a very shallow reading of Codd or you seriously believe that walking one's way out of the ocean is just as good a way to avoid drowning as swimming is. Received on Sat Aug 15 2009 - 20:12:12 CEST

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