At risk of distracting from the discussion at hand:
If you feel that descriptive data can only be modeled with relations, then you can never get XML to work for you. It is not
relational, it is strictly
hierarcical. It does not and, by design, cannot support the relational
model. Do not
confuse XML with a database. It is -not- designed to be a database. The
few databases that
use XML are all hierarcical XML is designed to model the structure of documents. If you want to create a database, use a database language. XML is what you use in the report.
Well strictly speaking XML can be used to model/serialise graph structures, so while it is hierarchical, and the syntax does require a single root, there's no reason why this can't become a graph (or other) structure once parsed.
There's been a lot of activity (unsurprisingly) in serialising XML into and out of databases. In general it's not an easy problem to solve, but it can be done.
Also, I'd have to disagree with the statement that 'the few databases that use XML are all hierarchical'. Current XML-aware databases fall into two categories: XML-Enabled Databases (i.e. relational databases that have added XML features) and Native XML Databases (i.e. those specifically designed to store XML). Databases in the first category do tend to be primarily relational in nature. Those in the second are a mixture: they model the XML document internally, and this might be achieved using a relational model.
Cheers,
L.
-- Leigh Dodds, Research Group, Ingenta | "Pluralitas non est ponenda http://weblogs.userland.com/eclectic | sine necessitate" http://www.xml.com/pub/xmldeviant | -- William of Ockham