# Re: Relation subset operators

Date: Thu, 04 Jun 2009 18:16:48 -0700

Message-ID: <79sg25d9pfp40tgdtk3nc2du47bth815fu_at_4ax.com>

cimode_at_hotmail.com wrote:

[snip]

>> 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.

[snip]

Sincerely,

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