As far as I know, there are no nulls in the Serial Number column. You do bring up a good point though. I took another look at my criterion for the query and discovered that I should have typed:

Where VehiclesToUpdate.SerialNumber Is Null rather than where VehicleDetail.Serial.Number Is Null. When I executed it this way, I returned 157,511.

As for the stats, total records in the VehicleDetail are 209,809. However VehiclesToUpdate returns 177,303 which leaves 32,506 NOT 157,511.

This is my third attempt to post. I hope it works this time.