Re: Question about Date & Darwen <OR> operator

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 Fri Sep 02 2005 - 19:11:31 CDT