Re: Question about Date & Darwen <OR> operator

From: paul c <toledobythesea_at_oohay.ac>
Date: Fri, 02 Sep 2005 23:35:15 GMT
Message-ID: <Tk5Se.66722$Hk.18346@pd7tw1no>

Mikito Harakiri wrote:
> Marshall Spight wrote:
>

>>Mikito Harakiri wrote:
>>
>>>Assuming domains x in {1,2} and y in {a,b} what is the result of
>>>
>>>{(x=1)} <OR> {(y=a)}
>>
>>As I understand it, the result would be:
>>
>>x y
>>---
>>1 a
>>1 b
>>2 a
>>
>>
>>which is
>>{ (x, y) | x = 1 or y = a }

>
>

>>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 assume you're referring to TTM page 56, which i misinterpreted for a long time.

i believe the answer is not the above, rather there are 4 tuples, two with x=1, ie. one for each of all possible values of the y domain and two y=a, ie. one for each of all possible values of the x domain.

i posed another one like this a few months ago and only just realized i had it wrong, even though somebody else had agreed with me!

i believe that ttm assumes that domains are finite, or at least that relations are finite which would seem to imply finite domains.

p Received on Fri Sep 02 2005 - 18:35:15 CDT