Re: Question about Date & Darwen <OR> operator

From: Marshall Spight <marshall.spight_at_gmail.com>
Date: 2 Sep 2005 17:11:31 -0700
Message-ID: <1125706291.163098.312610_at_g14g2000cwa.googlegroups.com>


Mikito Harakiri wrote:
>
> >From http://c2.com/cgi/wiki?RelationalAlgebra
>
> a OR b : An extended form of union; if the headings of the operands
> differ, then "missing" attributes take on all possible values. Thus the
> result may be very large or even infinite. When the operands have the
> same heading, then this is the same as a traditional SQL UNION, except
> that all duplicates are always removed.
>
> This informal description matches the other alternative. What is the
> formal definition?

I haven't seen one. D&D don't really do anything with formal methods that I've seen.

I would propose something like

Given
A:(a:Ta,ab:Tab)
B:(b:Tb,ab:Tab)

A <OR> B = { (a,ab,b) |
  ((a,ab) in A cross product Tb)
  union
  ((b,ab) in B cross product Ta)
}

Is that sufficiently formal? What would constitute a sufficiently formal form?

Marshall Received on Sat Sep 03 2005 - 02:11:31 CEST

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