:block.dept_code := null;

:parameter.old_unit := :block.unit;

if :block.unit <> :parameter.old_unit then
   :block.dept_code := null;
end if;

declare
  l_dept_code   dept_codes.dept_code%type;
begin
  select d.dept_code
    into l_dept_code
    from dept_codes d
    where d.unit = :block.unit
      and d.dept_code = :block.dept_code;
exception
  when no_data_found then
    message('Dept code and unit do not match');
    raise form_trigger_Failure;
end;