Re: On Formal IS-A definition

From: Tegiri Nenashi <>
Date: Thu, 6 May 2010 15:12:58 -0700 (PDT)
Message-ID: <>

On May 6, 7:32 am, paul c <> wrote:
> ... Regarding another aspect of the Information Principle, I'd like to ask
> whether an SuppliersParts relation such as:
> S# P#
> S1 P1
> S1 P2
> S2 P3
> has the same information as one with a domain PS# that is a set of part
> numbers:
> S# PS#
> S1 {P1,P2}
> S2 {P3}
> If they have the same information, does that mean they are equivalent,
> ie., each implies the other?  If so, is there a practical need for a
> relational equality operator (as opposed to equality between domain values)?

I would suggest that you need to amend your database schema with binary set membership relation, which in your case is something like

Sets = [m _s]

        P1  "{P1,P2,P3}"
        P2  "{P1,P2,P3}"
        P3  "{P1,P2,P3}"
        P1  "{P1,P2}"
        P2  "{P1,P2}"
        P3  "{P3}"


You may also want an unary empty set relation

EmptySet = [_s]


This framework eliminates any need for nested relations (aka non 1- NF).

Are you asking whether set equality join is a legitimate relational operator? Conventional wisdom is that it is not fundamental because it can be expressed in terms of the others... Received on Thu May 06 2010 - 17:12:58 CDT

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