Multiple databases in a single query

I need to query multiple databases in a single query.
e.g.
Database d1 contains table t1 with column id
Database d2 contains table t2 with column id
I need a query like
SELECT * FROM t1, t2 WHERE t1.id = t2.id;
is this possible in jdbc? if yes can anyone help with some sample code?
thanks,
Ashish Saraf

It is not possible using the JDBC API to query tables in two or more databases using a single query. The reason is that a JDBC Connection is made to a single data source.
However, some databases support the idea of federations or linked tables. What you do is inside the database manager link or federate a table that is present in another DBMS. Then, pose a single JDBC query to the one DBMS which will have its own table and links to the external tables.
Another approach is to use a special JDBC driver with a built-in integration engine that will perform a query across multiple databases. In effect, the system poses separate queries to the different databases and integrates and joins the data together for you automatically on the client-side. That is much easier than doing it yourself in code, but that is also possible for simple queries.
As part of my research program, we have released a shareware version of the UnityJDBC driver that can query multiple databases using a single SQL query. For more information, see:
http://www.unityjdbc.com
http://www.cs.uiowa.edu/~rlawrenc/research/projects.html
Sincerely,
Dr. Ramon Lawrence
Assistant Professor
Department of Computer Science
University of Iowa
[email protected]

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