simple trigger

Hi,

I want to update one of the columns with the sum of 2 other columns on the same table.

I have written a trigger, which works fine with insert statements but on update, it's mutating. Whats the round about way of doing it.
Egs:
/******works fine************/
create or replace trigger t_nbn1
after insert on nbn1
begin
update nbn1
set c3 = c1 + c2;
end;
/*********************MUTATING PROBLEM****/
create or replace trigger tu_nbn1
after update on nbn1
for each row
begin
update nbn1
set c3 = c1 + c2;
end;
*************************************************
Badrinath

Refer:
Note: 74859.1
Note: 37906.1
Note: 135051.1

From the metalink...

Sam

Will the following do?

create or replace trigger trig_t1
before insert or update on t1
for each row
begin
:new.c3 := :new.c1 + :new.c2;
end trig_t1;

-amar

create or replace trigger xy
before insert or update on ccc
for each row
begin
If inserting then
:new.x := :new.y+:new.z;
end if;
If updating then

:new.x := :old.y+:old.z;
end if;
end;