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Michael Kay <michael.h.kay@ntlworld.com> 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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