Updating a timestamp when a column value changes

Question

Hi,
currently i have a table that stores an assigned_to column and an assigned_on column.
what i would like to do is update the assigned_on column with the systimestamp when the old value of the assigned_to column is not the same as the new value.
i have tried using a trigger to fire when :new.assigned_on != :old.assigned_on, however, i experience the mutating table problem.
i have looked around and tried using 3 triggers (before and after row and after statement) in conjunction with the package, however, have not been able to fix it.
can someone suggest an alternative, short of putting the the assigned_on value into a different table.
i already have a trigger that is copying the the row across to a log table, but i don't think that is causing the mutation errors.
your help and suggestions are greatly appreciated in advance.

Answer 1

SQL> create table t (assign_to integer, assign_on timestamp);
Table created.
SQL> create or replace trigger tr_01
  2  before insert or update of assign_to
  3  on t
  4  for each row
  5  when (new.assign_to != nvl(old.assign_to,0) and new.assign_to is not null)
  6  begin
  7   :new.assign_on := systimestamp;
  8  end;
  9  /
Trigger created.
SQL> insert into t values(null,null);
1 row created.
SQL> select * from t;
ASSIGN_TO ASSIGN_ON
SQL> insert into t values(1, null);
1 row created.
SQL> select * from t;
ASSIGN_TO ASSIGN_ON
        1 12.09.05 14:49:16.234000
SQL> update t set ASSIGN_TO = 1 where ASSIGN_TO = 1;
1 row updated.
SQL> select * from t;
ASSIGN_TO ASSIGN_ON
        1 12.09.05 14:49:16.234000
SQL> update t set ASSIGN_TO = 2 where ASSIGN_TO = 1;
1 row updated.
SQL> select * from t;
ASSIGN_TO ASSIGN_ON
        2 12.09.05 14:49:42.890000Rgds.