数据库树形结构，正反遍历
--从Root往树末梢递归
select level ,identity,pid from table_name
start with identity=475
connect by prior identity = pid
--从末梢往树ROOT递归
select level ,identity,pid,yylevel from table_name
start with identity=542
connect by prior pid = identity
原文：http://www.cnblogs.com/jingsha/p/5595992.html