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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.
Michael Kay
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