Creating a XML Schema for an existing database

Hi All,
       Currently studying XML Technologies at postgraduate level. I know
that you can use XML to replicate a database structure. I want to enforce
Data Integrity i.e. Entity integrity, Domain integrity, Referential
integrity and User-defined integrity. Can referential integrity be
maintained using Unique Values by the 'Key and Key References,' technique?

Please note I'm using the Apache Xerces validate tool. It does not seem to
catch a deliberate error I placed in it. Please excuse me for my ignorance
I'm still on my 'Learner Plates.'

Please see attached files.

Thanks Marco Mastrocinque