discuss two fundamental problems in information integration:
(1) how to answer a query over a public interface which combines data
from several sources and
(2) how to create a single database conforming to the public interface
which combines data from several sources.
I consider the case where the sources are relational databases, where
the public interface is a public schema (a specification of the format
of a database), and where the sources are related to the public schema
by a mapping that is specified by constraints.