RE: Interdependence between fields

My question:
How can I restrict that Canada is allowed with Montreal and Toronto, USA is
with New York, Washington or Boston and Mexic is allowed only with Veracruz.



You can't. And I'm really not sure it would a good idea if you could. There
are some things that are data, and some that are metadata, and to my mind,
the fact that Montreal is in Canada is data, not metadata.
 
I don't think you'd want your schema to have built-in knowledge that there
are no direct flights from Manchester to Dallas, after all.
 
Michael Kay
http://www.saxonica.com/