Hi Nigel,
SQL Passthrough goes directly to SQL Server and does not pass Go!. This means that it is totally separate from your VFP data.
If you need to process VFP data in SQL Server, you will need to copy your VFP data into a SQL temporary table first, although with 100k rows, this could be a problem.
Alternatively, you could download all the necessary data from SQL into a local VFP table and the process the data together, probably an equally unweildy option.