have a try:
create or replace trigger test1
before update on aa
for each row
declare
-- local variables here
begin
if (:new.fd2 is null) then
update aa set :new.fd2=:new.fd1;
end if;
end test1;
create or replace trigger test1
before update on aa
for each row
declare
-- local variables here
begin
if (:new.fd2 is null) then
update aa set :new.fd2=:new.fd1;
end if;
end test1;
create or replace trigger test1
before update on aa
declare
-- local variables here
begin
if (:new.fd2 is null) then
:new.fd2 :=:new.fd1;
end if;
end test1;