I am giving you an example.

Table Employee has following columns
empcode,
ESIno,
..
..

in above said table, both fields are individualy
ok for Uniq Key, but only EmpCode can be
primary key, because ESI no. may
be null for some employees. But whenever it will be updated
it must be uniq value.
PF no. is also example for Uniq Key.


P. Soni