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$Hk.18346_at_pd7tw1no>
>
>
>
> a OR b : An extended form of union; if the headings of the operands
> differ, then "missing" attributes take on all possible values. Thus the
> result may be very large or even infinite. When the operands have the
> same heading, then this is the same as a traditional SQL UNION, except
> that all duplicates are always removed.
>
> This informal description matches the other alternative. What is the
> formal definition?
>
Date: Fri, 02 Sep 2005 23:35:15 GMT
Message-ID: <Tk5Se.66722$Hk.18346_at_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 Sat Sep 03 2005 - 01:35:15 CEST