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It is better to be very accurate in what is discussed.
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
>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.
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