Re: Question about Date & Darwen <OR> operator
From: paul c <toledobythesea_at_oohay.ac>
Date: Sat, 03 Sep 2005 00:33:51 GMT
Message-ID: <Pb6Se.67282$Hk.4881_at_pd7tw1no>
>
>
> I haven't seen one. D&D don't really do anything with formal methods
> that I've seen.
>
> I would propose something like
>
> Given
> A:(a:Ta,ab:Tab)
> B:(b:Tb,ab:Tab)
>
> A <OR> B = { (a,ab,b) |
> ((a,ab) in A cross product Tb)
> union
> ((b,ab) in B cross product Ta)
> }
>
> Is that sufficiently formal? What would constitute a
> sufficiently formal form?
>
>
> Marshall
>
Date: Sat, 03 Sep 2005 00:33:51 GMT
Message-ID: <Pb6Se.67282$Hk.4881_at_pd7tw1no>
Marshall Spight wrote:
> 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
>
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 }