Thank you, Joe.
No, you can pick the fields to be updated. Fields that are not included will not be updated. However, note that this is not the same as merging data per user per field, with a preference for "more data". The update has to "call its shot", in effect saying "these are the fields I am updating". This is necessary to allow a field to be overwritten with an empty value (for example, a phone number being updated as disconnected).
OK, so to avoid overwriting a field, all I have to do is make sure that my CSV file does not have that field. Conversely, if I include a field, then that field will be overwritten, even if it happens to be empty for a particular patron. Do I have this right? Cab Vinton, Director Sanbornton Public Library Sanbornton, NH