感谢两位极客大侠的精彩讲解：https://www.cnblogs.com/linjiqin/archive/2013/06/24/3152674.html
https://www.cnblogs.com/king-xg/p/6794562.html
Oracle 树操作(select…start with…connect by…prior)
原文：https://www.cnblogs.com/xianfengzhike/p/9250410.html
