I want to chirp up and agree with
tye, but add that it might be a good idea to have an ID type column as well, with values like "SSN", "TIN", "Dummy", etc-- that way if you ever need to you can key on the combination of the two fields, rather than the single ID number (since they recycle SSN you might have cause to label one "Expired SSN"). Also, I wouldn't worry too much about the length, because a good database should make it possible to enlarge the field size without changing any existing data.
