存储过程取不到值怎么办

存储过程取不到值怎么处理？
select nvl(c.c_id, '0')
  into warn_id
  from t_config c
  right join (select '0' from dual) tt on 1 = 1
  where risk_val between c.start_val and c.end_val;

warn_id,risk_val都是过程中定义的变量，但是risk_val可能不在config表中start_val与end_val之间，返回空行，为了避免这种情况，我来了个right join (select '0' from dual) tt on 1 = 1,但是还是错误啊，依然返回空行，报错，大侠们看看，怎么改改啊

------解决方案--------------------

SQL code

select nvl(c.c_id, '0')  into warn_id  from t_config c  right join (select '0' from dual) tt on 1 = 1 and risk_val between c.start_val and c.end_val;--你加入到了where中，right join对于tt失效了