oracle 递归查询父子关系记录
select lpad(' ',4*(level-1))||t.project_id project_id,t.parent_project_id
from PC_PROJECT_ASSOC t
start with project_id='xRFv29ZYbE'
connect by prior project_id = parent_project_id
strat with 指定层次开始的条件，即是说满足这个条件的行即可以做为层次树的最顶层
connect by prior指层之间的关联条件，即什么样的行是上层行的子行（自连接条件）
