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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
> relationships?".
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 cowan@ccil.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."
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