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Date: Mon, 06 Jun 2005 20:22:29 +0200
From: mAsterdam <mAsterdam@vrijdag.org>
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Newsgroups: comp.databases.theory
Subject: Re: Translating constraints to  RM Terms
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Xref: dp-news.maxwell.syr.edu comp.databases.theory:31265

Kenneth Downs wrote:

> I'm wondering if some RM theorist might express something for me.
> 
> Consider table X with columns A and B.  There is a constraint that A must be
> less than B.  While we discuss such constraints every day, I realize I do
> not know how to express such constraints in relational terms.
> 
> For instance, A is actually a domain, as is B, but the constraint A < B must
> in fact be part of the very definition of the domain A, and here we are
> defining one domain in terms of another.  I realized I have not seen this
> disccussed in the year or so I have been a regular here.  Is defining one
> domain in terms of another allowed and considered trivial, no big deal?  Is
> it actually not allowed and this is some SQL alteration of true RM?

The constraint 'A < B' suggests to me that there is
'something' of A in B or the other way round. In this
case it is easy to engage some isolation.
The base table (say W) should just contain A and C, C being a positive 
number. X becomes a view with A = W.A and B = W.A + W.C

That this isolation is possible in this example
should not be taken to mean that it is in all cases
you have in mind, though.

My 2 Eurocents.