XML vs relational database

Hello,

In an XML document, querying the data with XPath may inform about both
the data _and_ the structure of the tree, the design of the document (for
instance, in a data-oriented document, I can ask for the name of the
elements, the number of elements and attributes in each "record"-sub tree,
its depth, etc.). In a relational database, a standard query allows to
retreive data, but does not allow to investigate the design of the
database (on some DBMS I can however query the administration database for
properties of the user databases, or I can parse the result of a query for
extracting the name and number of columns, but it is indirect).

Can I say that? And, if yes, what is the name of this property of trees as
data structures, as opposed to relational databases?

Best,
Sylvain

-- 
Sylvain Loiseau

http://www.limsi.fr/~sloiseau
 
On peut pratiquer objectivement, c'est-à-dire impartialement,
une recherche dont l'objet ne peut être conçu et construit
sans rapport à une qualification positive et négative, dont
l'objet n'est donc pas tant un fait qu'une valeur.
 
Canguilhem, /Le normal et le pathologique/, p. 157