create table emp_hire as
selectempno,mgr,ename from emp
select * from emp_hire
这些数据存在着层次关系
select
lpad('*',level,'*')||e.ename ename,
e.empno,
mgr fa_id,
(select ename from emp_hire where empno=e.mgr) fa_name
from emp_hiree
START WITH MGR IS NULL
CONNECT BY PRIOR EMPNO = MGR
层次是一棵树， 又如同一个家族图谱
每一个子节电只有一个父节点；
每一个分支，最末端是叶节点；
SELECT
ename Employee,
CONNECT_BY_ISLEAF IsLeaf,
LEVEL lev,
SYS_CONNECT_BY_PATH(ename, '/') Path
FROM emp
START WITH MGR IS NULL
CONNECT BY PRIOR EMPNO = MGR
检查一个层次是否存在闭循环
CONNECT_BY_ISCYCLE
找出根节点
CONNECT_BY_ROOT
SELECT
ename Employee,
CONNECT_BY_ISCYCLE,
CONNECT_BY_ROOT ename
FROM emp
START WITH MGR IS NULL
CONNECT BY NOCYCLE PRIOR EMPNO = MGR
层次查询的过滤条件
SELECT
ename Employee,
CONNECT_BY_ISLEAF IsLeaf,
LEVEL lev,
SYS_CONNECT_BY_PATH(ename, '/') Path
FROM emp
where level<3
START WITH MGR IS NULL
CONNECT BY PRIOR EMPNO = MGR
Restriction on LEVEL in WHERE Clauses In a [NOT] IN condition in a WHERE clause, if the right-hand side of the condition is asubquery, you cannot use LEVEL on the left-hand sideof the condition. However, you can specify LEVEL in a subquery of the FROM clause to achieve the same result. For example, thefollowing statement is not valid:
SELECT employee_id, last_name FROM employees
WHERE (employee_id,LEVEL)
IN (SELECTemployee_id, 2 FROM employees)
START WITHemployee_id = 2
CONNECT BY PRIORemployee_id = manager_id;
But the following statement is valid because it encapsulates the querycontaining the
LEVEL information in the FROM clause:
SELECT v.employee_id, v.last_name, v.lev
FROM
(SELECT employee_id,last_name, LEVEL lev
FROM employees v
START WITHemployee_id = 100
CONNECT BY PRIORemployee_id = manager_id) v
WHERE(v.employee_id, v.lev) IN
(SELECT employee_id,2 FROM employees);
SIBLINGS的排序
SELECT
ename Employee,
CONNECT_BY_ISLEAF IsLeaf,
LEVEL lev,
SYS_CONNECT_BY_PATH(ename, '/') Path
FROM emp
START WITH MGR IS NULL
CONNECT BY PRIOR EMPNO = MGR
ORDER SIBLINGS BY ename desc
注意：ORDER SIBLINGS By一定与start with 与 connect by一起使用