Input from 2 different Database Connections

Hi,

What I am trying to do is get some ID numbers from a database on one connection.
Then SELECT some rows from a table on another database connection using the values input from the first.

Essentially I get :
12345
23456
34567
etc
from the first connection,
and then I want to select values from the second connection, which also has an ID field, if the ID is in the results from the first connection.

FYI - The 2 IDs are both the same datatype and length.

I realise that I could get the all the data in from both connections and then do a join,
but I am dealing with millions of rows and that is far to time consuming for this application.