ORACLE 10g新增了阶层查询操作符PRIOR，CONNECT_BY_ROOT
■PRIOR
阶层查询的CONNECY BY condition的条件式需要用到PRIOR来指定父节点，作为运算符，PRIOR和加(+)减(-)运算的优先级相同。
■阶层查询
语法：START WITH condition CONNECT BY NOCYCLE condition
START WITH 指定阶层的根
CONNECT BY 指定阶层的父/子关系
NOCYCLE 存在CONNECT BY LOOP的纪录时，也返回查询结果。
condition ... PRIOR expr = expr 或者 ... expr = PRIOR expr
例：
CONNECT BY last_name != 'King' AND PRIOR employee_id = manager_id ...
CONNECT BY PRIOR employee_id = manager_id and
PRIOR account_mgr_id = customer_id ...
■CONNECT_BY_ROOT
查询指定根的阶层数据。
■CONNECT BY子句的例子
通过CONNECT BY子句定义职员和上司的关系。
SQL>SELECT employee_id, last_name, manager_id
FROM employees
CONNECT BY PRIOR employee_id = manager_id;
EMPLOYEE_ID LAST_NAME MANAGER_ID
----------- ------------------------- ----------
101 Kochhar 100
108 Greenberg 101
109 Faviet 108
110 Chen 108
111 Sciarra 108
112 Urman 108
113 Popp 108
200 Whalen 101