Preventing update of a single column

Question

Hi All,
I'd like to ensure that values in a certain column are never updated, not even by the table owner. I have the following solution, but I'm interested to see if anyone out there has better alternatives. A check constraint solution would be ideal, but I'm not sure if it's possible.
SQL> create table t (pk number not null primary key, col varchar2(2) );
Table created.
SQL>
SQL> create or replace trigger t1
  2  before update of pk on t
  3  for each row
  4  begin
  5    raise_application_error
  6      ( -20000, 'You cannot update PK values in this table.' );
  7  end;
  8  /
Trigger created.
SQL>
SQL> -- these commands should work
SQL>
SQL> insert into t values (1, null);
1 row created.
SQL> insert into t values (2, null);
1 row created.
SQL>
SQL> update t set col = 'xx';
2 rows updated.
SQL>
SQL> -- this one should fail
SQL>
SQL> update t set pk=pk+1;
update t set pk=pk+1
ERROR at line 1:
ORA-20000: You cannot update PK values in this table.
ORA-06512: at "SCOTT.T1", line 2
ORA-04088: error during execution of trigger 'SCOTT.T1'

Answer 1

No need to disable anything:
SQL> insert into t t1
  2  (pk, col)
  3  select pk + 100
  4        ,col
  5  from t;
2 rows created.
SQL> select *
  2  from t;
                  PK CO
                   1 xx
                   2 xx
                 101 xx
                 102 xx
SQL> delete t
  2  where pk < 100;
2 rows deleted.
SQL> commit;
Commit complete.
SQL>