Re: Modelling Disjoint Subtypes
Date: Sun, 25 Mar 2007 12:05:31 GMT
> 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 ?"
> One often wants to consider all the different sub-entities together.
You just punched the tar baby. Entity? What entity?
> 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.
All of that is irrelevant. The specific constraint was chosen to maximize use of foreign key constraints and to minimize general WFF usage. That is all.
> 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?
In the general case, one will have both in any case due to the magic of views.
> You asked about the relevance of disjoint subtypes; I was
> pointing out how the construct appears in a wide variety
> of computational models.
Mutual exclusion or disjointedness is a concept. Thus one would expect to find that concept expressible one way or another in every computational model. Received on Sun Mar 25 2007 - 14:05:31 CEST