select level,employee_id, manager_id, first_name, last_name--level是伪列，代表位于树的第几层
from MORE_EMPLOYEES
start with employee_id = 1 --定义了层次化查询的起点
connect by prior employee_id = manager_id ;--表示父节点的employee_id和子节点的manager_id之间存在关系

7.5 层次化查询