SQL> create table test (code number, name varchar2(20));

Table created.

SQL> insert into test (code, name) values (100, 'Littlefoot');

1 row created.

SQL> create or replace trigger test_update
  2    before update on test
  3    for each row
  4  begin
  5    if :new.code <> :old.code then
  6       raise_application_error(-20001, 'Error: new code <> old code');
  7    end if;
  8  end;
  9  /

Trigger created.

SQL> select * from test;

      CODE NAME
---------- --------------------
       100 Littlefoot

SQL> update test set
  2    name = 'Bigfoot';

1 row updated.

SQL> update test set
  2    code = 200,
  3    name = 'Middlefoot';
update test set
       *
ERROR at line 1:
ORA-20001: Error: new code <> old code
ORA-06512: at "SCOTT.TEST_UPDATE", line 3
ORA-04088: error during execution of trigger 'SCOTT.TEST_UPDATE'


SQL>