Charles has made the most important statement on this (enforce database
independance). Just two additional comments:
- the mistake was made at the beginning and its very simple to avoid it: you
must not use any table (or PF) in any programm, only use Views (LFs)
- if the PO-Numnber would be part of a key in some table the situation would
be complicated a lot. You could avoid this wit a simple rule: you must not
use semantic keys.