references on bio DB integration

[Chris Overton]

I'm looking for references to work describing the integration of multiple
biological databases (well >= 2 at least).  I'm interested in examples where a
federated schema was developed that supports multi-database queries.  I'm
especially interested in examples of distributed database systems with a
federated schema.  However, I'll take any and all references to systems that
support multi-database queries.  Examples of types of systems:

1 -- Uniform interface to multiple databases.

2 -- Monolithic database containing multiple, independent, unintegrated
schemas.

3 -- Monolithic database containing multiple, integrated schemas (federated).

4 -- Distributed database system without federated schema.

5 -- Distributed database system with federated schema.

etc.

Thanks for your help.

Chris Overton