Perhaps this should be a text field with an auto-entered calculation. That will allow you to import data into the field and yet new records will compute the expected value.
account_number field in my customers table provides a unique account number that comprises the last 3 digits of the customer id number and the first three letters of the surname
is not guaranteed to be unique and I personnally would not use such a field as a match field in relationships.
A straight serial number is much simpler and far safer.
I do have unique number only serials ineachtable - this account number format is purely because of the imported data having a similar format...
I did try auto enter calc, but it doesnt seem to create the account number when I run the script...
Your scritp may have to set the field instead of relying on the auto-enter calc. Auto-enter calcualtions do not automatically update when you modify a field that they reference that comes from a different table or record.
You may also want to not import the data into this field. Import it into a separate text field and use a calculation in a script or Replace Field Contents to load the fields that the calcluation uses to produce this value.
Ah I see......I had thought of your second suggestion, but wasnt sure if there was another way...