Re: Question about Date & Darwen <OR> operator

"paul c" <toledobythesea_at_oohay.ac> wrote in message news:Pb6Se.67282$Hk.4881_at_pd7tw1no...
> the bit on pg 56 ...
> Hs = Hr1 union Hr2
> Bs = {ts: exists tr1 exists tr2
> ( ( tr1 in Br1 or tr2 in Br2) and
> ts = tr1 union tr2 }

If the above is indeed the <OR> definition, then I do not understand how it handles relations with the same header..

Let a tuple be a set of triples <A, T, v> (attribute, type and value). Further, let r1 and r2 be relations with the same header H. Now, what would be the result of tr1 union tr2 ? Obviously it's not a relational tuple any more because there are two attributes with the same name in the union. E.g.

tr1 = { <x,int,4>, <y, char,'a'}
tr2 = {<x,int,5>, <y, char,'b'}

tr1 union tr2 = { <x,int,4>, <y, char,'a'>, <x,int,5>, <y, char,'b'> }

What am I missing ?

>
> ... D & D call it a formal definition. i had the impression they wanted
> to use <AND> and <OR> to define the other operators, such as product from
> that starting point. personally, i like the connection with ordinary
> English, even if i'm often getting the result of <OR> wrong! (maybe that's
> why i like it since so many people i know get the English one wrong too,
> eg. "i'll see you today or tomorrow" usually means neither!)
>
> i was wondering - if you started with 'cross product' instead of <OR>, how
> would you define 'cross product'?
>
> thanks,
> pc
Received on Sat Sep 03 2005 - 14:30:27 CDT