A method includes receiving a query that is unstructured for a plurality databases; determining a service requirement fulfilled by the databases from the terms of the query; determining, for each of the databases, a service requirement score for the database; selecting one of the databases based on the service requirement scores; determining, from the query, parameter values for the service requirement; providing the parameter values for a search operation for the selected database and receiving database search results responsive to the parameter values and the search operation; determining, from the database search results, a search constraint for the search query; revising the search query to include the constraint and providing the revised search query to a search engine; and providing search results identifying resources that the search engine determined to be responsive to the revised query.