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Michael Kay <michael.h.kay@n...> writes: >> At one time hierarchical and relational databases were competing technologies, >> but overall the relational model, and relational databases, won out. The >> relational model is now more developed and generally accepted to be superior >> to the hierarchical model for most uses. (Please correct me if I'm wrong or >> oversimplifying here.) > You are over-simplifying, because there has never been a single hierarchical > model for databases. Most of the database textbooks equate "the hierarchical > model" to the IBM IMS product, and most of the weaknesses of that generation > of technology are nothing to do with its data model. The biggest shift from > the hierarchic and network-model databases to the relational model was the > move from procedural DMLs to declarative query languages, and of course XPath > and XQuery (and OQL before them) prove that it's perfectly feasible to use a > declarative query language over hierarchies and networks. In fact, the power > of these languages is greater than that of the relational calculus because it > extends to recursive queries. > > Probably the greatest weakness of XML as a data model for databases is that it > doesn't provide a coherent way of modelling the non-hierarchical relationships. > But that's a weakness of the relational model too. I'm having a hard time parsing this. Did you perhaps mean the inverse; that the relational model has a hard time modeling hierarchical relationships? Or is this a general comment about the difficulties in modeling for the relational model? If it's the latter I'd disagree; just about anyone can at least do a first normal form model. That may not get you real far, but tools abound as do training, books and tons of best practices to fall back on. A more general comment/question: it recently occurred to me that it is likely possible to model any XML Schema as a relational schema (proof of this theorem is left as an exercise for the reader ;-)? Don't know what that gets you, but as I've said at least the tools abound...
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