Assuming that you know the order for importing the files. I would import file 1 into a temporary table. Then append this temporary table to another table e.g. table_data using the field 1 as the primary key. Now we must assume that there maybe more than one record for a value in field 1, so we must update the table_data with the data in our temporary table.
For each other file we need to delete the temporary table then import, append and update.