Re: Relationships [was RE: James Clark: XML versus theWeb]
> In your messages I see the words "relationship" and "modeling" appearing repeatedly. That seems to be a key concept. > > Consider this XML snippet: > > <BookStore> > <Book> > ... > </Book> > <Book> > ... > </Book> > </BookStore> > > That snippet shows a relationship between BookStore and Book; namely, BookStore consists of Books. Is that an example of the kind of relationships you are talking about? That's one kind of relationship (what UML would call an aggregation relationship, represented here by hierarchic containment). Other common representations of relationships in XML are via ID/IDREF (that is, primary key / foreign key), and via cross-document URIs. There are other mechanisms, for example XSD and XSLT make heavy use of QNames. XSD also allows relationships to be described on any data type, or on composite key values, using xs:key/xs:keyref. A general problem is that there are many different mechanisms for describing relationships in XML, and none of them has particularly expressive semantics. Michael Kay Saxonica
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