> To take a simple example, the word 'schema' -- I have an understanding of 
> what this means in the context of XML, but it clearly also has a more
formal 
> meaning, as it can be applied to database design as well.  So what's the 
> definition of 'schema'?

A schema is essentially a data type, every database / document is an
instance of its schema, and the schema describes the set of all permitted
databases / documents.

> My question is, are there any books or online documents dealing with the 
> theory of 'information stuctures'?  Some kind of description of the ways
in 
> which information may be structured, and the advantages of doing it a 
> particular way?

[MHK>]  Yes! Though most of the ones I know of are written in the "database"
context rather than the "document" context. Some are very academic /
mathematical / philophical, some more oriented to the practitioner  [MHK>]
One of the best in my view, but very hard to get now, is Bill Kent's "Data
and Reality" 
Mike Kay  


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