Re: Linkbases, Topic Maps, and RDF Knowledge Bases -- help me

[Uche Ogbuji <uche.ogbuji@fourthought.com>]

> Let me concentrate only on relationships, w/o special focus on RDF.
> Regarding relationships: Binary relationships are quite easy --
>
> 1. Ordered 1:n binary relationships in XML schema -> using parent-child
> 2. Unordered n:1 relationships in XML schema -> using IDREF, we need IDREF
> to identify the target type, somewhat similar to what DSD had in mind.
> 3. Ordered n:m relationships (ordering on one side) -> using IDREFS, if
> IDREFS identify target type.
> 4. Unordered n:m relationships -> using what is done in the relational
> model -> have one relation for each entity, and one relation for the
> relationship and have some foreign key constraints.
>
> Of these, 2 and 3 are not really supported in any schema language. Can we
> hope for them?? - probably there should be a light weight schema
language
> that allows us to add such things more easily :)
>
> Let us consider N-ary relationships (N > 2). Again there are two cases
--
> (a) one of the entities has cardinality of 1 (min of 0 and max of 1)
> (b) all entities have maximum cardinality > 1
>
> The general idea in relational model and object model for N-ary
> relationship is to have a new object/type/relation for the relationship.
>
> I think one common example of ternary relationship is the supplier kind
of
> relationship. It is something like (not very sure of the exact one)
> m suppliers supply n products to p cities.
>
> I think the following is allowed in ODMG:
>
> class supply {
>   relationship Supplier supplier;
>   relationship Product product;
>   relationship City city;
> }
>
> I made one statement about being theoretically able to represent a
> N-ary relationship without creating a new type for the relationship. But I
> think that is possible only if one of the entities have cardinality 1, and
> is *not* possible otherwise. I think this can also be thought in terms of
> representing multiple relationships, but I think the fact is it works.

Either I'm missing something, or you're still mixing up N-ary
relationships with binary relationships of greater than unit cardinality.

Your ODMG example, for instance, is most certainly not an N-ary
relationship.  The relationship is from supply to suppiler, from supply to
product and from supply to city.  There is no relationship between, say
city and product or vice versa.

Once again, ODMG cannot express N-ary relationships.


-- 
Uche Ogbuji                               Principal Consultant
uche.ogbuji@fourthought.com               +1 303 583 9900 x 101
Fourthought, Inc.                         http://Fourthought.com
4735 East Walnut St, Ste. C, Boulder, CO 80301-2537, USA
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