Re: Relation subset operators

From: Gene Wirchenko <>
Date: Thu, 04 Jun 2009 18:16:48 -0700
Message-ID: <> wrote:


>> Why do you bring in relation subtype (and what is it, actually)?
>A relation R1 de facto contitutes a type. A relation R2 using R1 as a
>domain of possible values (each relation value being a possible value
>for R2) and before applying R2 specific constraints makes R2 a subtype
>of R1. One could view a subtype as a declaratively constrained subset
>of tuples or alternatively as specialized subset of tuples. Since all
>tuples in R2 necessarily belong to R2, performing aggregation between
>R1 and R2 does make sense. For instance consider INT as a relation
>whose body includes all integers and ODD_NUMBERS as a relation that
>derives from INT. Nothing prevents to my knowledge performing set
>operations between INT and ODD_NUMBERS even though they are not
>strictly speaking of the same type.

     Sure they are. For all x IN ODD_NUMBER, x IN INT.



Gene Wirchenko

Computerese Irregular Verb Conjugation:

     I have preferences.
     You have biases.
     He/She has prejudices.
Received on Fri Jun 05 2009 - 03:16:48 CEST

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