项目中为了减少I/O流，采用构建虚拟表的方法来显示特殊的数据：
如达到下面的效果：
[html] select '1' as ID,'高' as 等级 from dual union all select '2','中' from dual union all select '3','低' from dual;
构建虚拟表简析
[html] select '1' as ID,'高' as 等级 from dual union all select '2','中' from dual union all select '3','低' from dual;