declare 
  object_exists exception;
  except_01451  exception;
  except_01442  exception;

  pragma exception_init (object_exists , -01430);
  pragma exception_init (except_01451  , -01451);
  pragma exception_init (except_01442  , -01442);
begin 
    execute immediate 'ALTER TABLE MONITORAMENTO_CRZ MODIFY (EMPRESA_ID NULL)';

  exception 
    when object_exists  then null;
    when except_01451  then null;
    when except_01442  then null;
end;