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[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] RE: A standard approach to glueing together reusableXML frag
 > Hierarchical model had to represent all relationships using > parent-child. There was no concept of foreign keys. Hence > redundancy cannot be avoided, if you want to represent m:n > relationships. There was never a single definition of the hierarchical data model. Most writers equated it with the model implemented by the IBM product variously known as IMS or DL/I. Other writers (incorrectly) use the term to embrace the network data model (Codasyl) as well. I was never a fan of hierarchical databases myself (I worked extensively with Codasyl databases) but the statement that "redundancy cannot be avoided" is quite wrong. I've just been re-reading the relevant chapter from Tsichritzis and Lochovsky's "Data Models" (1982) which has an extensive discussion of the various techniques developed by vendors and users to support m:n relationships without redundancy: the most comprehensive solution being "spanning trees" which allowed multiple hierarchic views over the same data records. And although "foreign keys" were not part of the model, they were widely used in practice at the application level (just as they are in XML). The solutions seem rather ad-hoc (I said I'm not a fan), but it's quite wrong to say that they don't exist. It would actually do us no harm as a community to relearn some of this stuff. XML is hierarchical for a very good reason: it is optimized for data interchange. Data that is sent from A to B has to be encoded as a sequence of bits, and hierarchies lend themselves well to such serialization. This absolutely gives you a design challenge because the models that you get from your data analysis are graphs rather than trees. We certainly need a much more mature understanding of the methodology of translating between the graph object models that come out of data analysis and the hierarchic representation of these models as XML, and I would love to see something that gives you the ability to get multiple hierarchic XML views over the same network data model. Michael Kay 
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