>But my problem was to find a way to really join a datatable in memory with tables in the backend. It seems that to dump keys in an in() clause in sql is the way to go. It works when you just have a set of keys, not sure how to deal with that in more complex situations.
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We have this same issue with VFP and SQL Server... VFP cursors to join with underlying tables. Very simple within VFP, but not so simple when the database engine is separate. Our solution was to send the cursor to SQL Server as a temporary table, which could then be joined with other tables. It looks like Access has some support for temporary tables, but I don't know the details of how it works.

http://office.microsoft.com/en-us/access-help/create-table-statement-HP001032220.aspx