[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
Precisely. It is better to be very accurate in what is discussed. Thank you. Larry Bradshaw At 09:15 AM 8/26/2003 +0100, Michael Kay wrote: > > 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 > > >----------------------------------------------------------------- >The xml-dev list is sponsored by XML.org <http://www.xml.org>, an >initiative of OASIS <http://www.oasis-open.org> > >The list archives are at http://lists.xml.org/archives/xml-dev/ > >To subscribe or unsubscribe from this list use the subscription >manager: <http://lists.xml.org/ob/adm.pl>
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