I have created a database using the same names you have described in the query, all I have is serial_number in one, and serial in the other. No other column names. They are set as indexed with duplicates allowed. I am doing this to try and replicate the problem you are having. I am entering the data myself, bogus data, but data that works. Not the exact same as your database, but enough of it so I can get a better idea of what is going on.