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Michael Kay scripsit:
> XML has a clear story on hierarchical relationships, but a very confused
> story on those that aren't hierarchical. There are lots of ways of doing it:
> URIs, IDREFs, XLinks, or just "unmarked" foreign keys. None of these is
> clearly definitive.
Only one of these (ID/IDREF) actually belongs to the model. The rest
belong to higher-level protocols, which could just as well be layered on
the relational model as the XML model.
> There are probably two design decisions in XML that are really hard to make
> correctly, yet very hard to change later. One is "what goes in a document?",
> the other is "which relationships should be represented as parent-child
XML was designed to model documents, which are the most complex form of
data, and where the answers to these questions are evident. Probably
if the answer isn't evident for a particular job of modeling simpler
data, you shouldn't use these features except in stereotyped ways.
See Henry Thompson's paper on various normal forms of XML.
John Cowan firstname.lastname@example.org www.reutershealth.com www.ccil.org/~cowan
Heckler: "Go on, Al, tell 'em all you know. It won't take long."
Al Smith: "I'll tell 'em all we *both* know. It won't take any longer."