Re: Modelling Disjoint Subtypes
Date: Sun, 25 Mar 2007 04:24:27 GMT
Message-ID: <%PmNh.2381$5E3.1104_at_trndny01>
"Marshall" <marshall.spight_at_gmail.com> wrote in message
news:1174785951.893574.149480_at_o5g2000hsb.googlegroups.com...
> On Mar 24, 11:26 am, "V.J. Kumar" <vjkm..._at_gmail.com> wrote:
> > "Marshall" <marshall.spi..._at_gmail.com> wrote
innews:1174761278.831402.100630_at_y66g2000hsf.googlegroups.com:
> >
> > > On Mar 24, 9:04 am, "V.J. Kumar" <vjkm..._at_gmail.com> wrote:
> >
> > >> In other words, under what circumstances, other than an attempt to
> > >> emulate object oriented viewpoint, "R <x, y>; R1 <super R, z>; R2
> > >> <super R, w>" is 'better' than just "R1<x,y,z>, R2<x,y,w>" ? What is
> > >> achieved by such decomposition ? Clearly, there is no data
> > >> redundancy because R1 and R2 are disjoint !
> >
> > > If there is no constraint separating R1<x, y> with R2,<x,y>, then
> > > they are *not* disjoint.
> >
> > When I said "because R1 and R2 are disjoint", I implied that there is a
> > constraint of course, e.g.: "R1 join R2 is_empty" or similar, as
there
> > would be with the three relvars !. Having dealt with that diversion,
> > back to the original question: "under what circumstances, other than
> > an attempt to emulate object oriented viewpoint, "R <x, y>; R1 <super
R,
> > z>; R2 <super R, w>" is 'better' than just "R1<x,y,z>, R2<x,y,w>" ?
What
> > is achieved by such decomposition ?"
>
> Okay.
>
> One often wants to consider all the different sub-entities together.
> If one has ten different disjoint types, and one wants to count
> them, having a table for the common attributes means the
> count() can be done with a single table, vs. a join of ten tables.
>
> On the other hand, if one has a query that needs both common
> and unique attributes, that query would require two tables vs.
> just one if we didn't have the common attributes in a supertype
> table. Anyone have any other considerations?
> >
> > I am not sure I understand the relevancy of your appeal to functional
and
> > OOP point of view.
>
> You asked about the relevance of disjoint subtypes; I was
> pointing out how the construct appears in a wide variety
> of computational models.
>
>
Isn't this discussion another variation of the "generalization specialization" discussion we have every now and then? It seems that at least once a month we get a visitor that wants to model that, and build some tables that are suitable for manipulating data.
Just asking. Received on Sun Mar 25 2007 - 06:24:27 CEST