So the tables are in different databases? You don't mention what type of database you're using, but in Oracle and SQL Server there is the concept of "Database Links" where you can query data from one database while logged in to another.
If something like this is available to you and you have a way of determining in the Main Report which database to "link" to for the subreport (whether you have a field in the data from the main report or you create formula that will do this) then you might be able to do the following:
1. Use a command in your subreport. Using Oracle syntax, it might look something like this (assuming the table name is the same in every database)
Select
Field1,
Field2,
...
Fieldn
from MyTable@{?DBLINK}
where Field1 = '{?Some Parameter}'
2. Create the two parameters - DBLINK and Some Parameter in the Command Editor (DO NOT create them in the subreport Field Explorer - they won't work with the command!)
3. In the main report, go to the subreport links. Select the field or formula in the main report that contains the database link information. Turn off "Select data in subreport based on field:". In the drop-down on the bottom left, select the {?DBLINK} parameter you set up in the command. Do the same thing for any other parameters you want to pass in from the main report.
This should work because parameters in a command are not really parameters in a database query sense, Crystal will just replace them with the value that is set for the parameter. However, I don't guarantee it will work as I have never actually tried doing this.
-Dell