Merging Databases

Hi all,
I am attempting to merge 3 databases: (i)
Clinicaltrials.gov<http://clinicaltrials.gov/> ,
(ii) Geonames <http://www.geonames.org/> , (iii)
FDA<http://www.fda.gov/Drugs/InformationOnDrugs/ucm135162.htm> based
on ontologies.

There are RDF Triples already defined for (i) http://linkedct.org/index.html
and there is already an ontology present for (ii)
http://www.geonames.org/ontology/ . However, there is no ontology present
for the FDA database. The field "Zip Code" is common for all the three
databases.

Could anyone suggest possibilities of how to merge the three databases,
based on ontologies?

Thanks,
Regards,
Amrapali J Zaveri