Could you describe what do you mean? My modules does support work with several databases (different dbh's) at the same time - and should support multiple DB types (MySql, SQLite, Postgree ...). But I have no idea how to possibly join two tables in different DB's (with JOIN or say sub query).

I think that's exactly what's being requested -- a join or sub query using multiple databases. Typically, you can rewrite a join as a subquery, or vise-versa, and databases typically handle two different styles of joins (sort-merge or correlated).

With a sort-merge, you take the results from each table, sort them on the fields that it's getting joined by, and then work your way through the lists, finding entries in each list that have those fields in common.

With a correlated query, you get the results from one table, then for each instance of the fields that you're trying to merge on, you query the other table.

Depending on the size of the two tables being joined, one or the other may be more efficient. (and of course, with a correlated query, if it's an equijoin, you can work from either table first.) Database tuning involves storing histograms of the fields that you're going to be joining on, so you have a clue which way is going to be the most efficient.

Oh -- and for those using Oracle on both ends -- look into what they call 'database links'.