Re: On Formal IS-A definition
Date: Thu, 6 May 2010 15:12:58 -0700 (PDT)
Message-ID: <c15f44c4-633b-48ad-9f7a-e2dd7c13ca8b_at_u20g2000pru.googlegroups.com>
On May 6, 7:32 am, paul c <toledobythe..._at_oohay.ac> 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 Fri May 07 2010 - 00:12:58 CEST