cross database queries?

I want to write a query like:

select clients.client.client_id, columnar.sales.total_sales, 
web.page_hits from clients, columnar, web
where clients.client_id = columnar.client_id
and  clients.client_id = web.client_id

in a system where 'clients' is actually one or more relational 
databases, 'columnar' is one or columnar databases, and 'web' is the 
Apache logs on one or more web servers.  The dbi driver would be 
configured to connect to the correct databases and filter web hits based 
on 'client_id'.

Has somebody written that already?

Thanks,
Andrew