# Re: On Formal IS-A definition

From: Tegiri Nenashi <tegirinenashi_at_gmail.com>

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:

;

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 Thu May 06 2010 - 17:12:58 CDT