我 们的树状属性一般都是在一条记录中记录一个当前节点的ID和 这个节点的父ID来实现。但是，一旦数据中出现了循环记录，如两个节点互为 对方父节点，系统就会报ORA-01436错误。10G中， 可以通过加上NOCYCLE关键字避免报错。并且通过CONNECT_BY_ISCYCLE属 性就知道哪些节点产生了循环：
drop table t;
create table t(cid int,
pid int);
truncate table t;
/*
1--
|--2
|--3
|--5
|--1(cycle)
|--6
|--4
11--
|--6
|--112
*/
-- tree number 1
insert
into t(cid, pid) values(1,null);
insert into t(cid, pid)
values(2,1);
insert into t(cid, pid) values(3,1);
insert
into t(cid, pid) values(4,1);
insert into t(cid, pid)
values(5,3);
insert into t(cid, pid) values(6,3);
--
another tree number 11
insert into t(cid, pid) values(11,null);
insert
into t(cid, pid) values(112,11);
insert into t(cid, pid)
values(6,11); -- node 6 in both trees
-- cycle node
insert
into t(cid, pid) values(1,5);
commit;
select * from t;
-- the path in row whose iscyle=1 tells the
cycle path
-- here is 1_3_5(_1)
select lpad(' ', 2 *
(level - 1)) || t.cid cid,
connect_by_root t.cid root,
connect_by_isleaf isleaf,
connect_by_iscycle iscycle,
substr(sys_connect_by_path(t.cid,'_'),2) path
from t
start
with t.pid is null
connect by nocycle t.pid = prior t.cid
order
siblings by t.cid;
-- 测试结果
SQL> select * from v$version;
BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Prod
PL/SQL Release 10.2.0.1.0 - Production
CORE 10.2.0.1.0 Production
TNS for Linux: Version 10.2.0.1.0 - Production
NLSRTL Version 10.2.0.1.0 - Production
SQL>
SQL> -- the path in row whose iscyle=1 tells the cycle path
SQL> -- here is 1_3_5(_1)
SQL> select lpad(' ', 2 * (level - 1)) || t.cid cid,
2 --connect_by_root t.cid root,
3 --connect_by_isleaf isleaf,
4 connect_by_iscycle iscycle,
5 substr(sys_connect_by_path(t.cid,'_'),2) path
6 from t
7 start with t.pid is null
8 connect by nocycle t.pid = prior t.cid
9 order siblings by t.cid;
CID ISCYCLE PATH
-------------------- ---------- ----------------------------------------
1 0 1
2 0 1_2
3 0 1_3
5 1 1_3_5
6 0 1_3_6
4 0 1_4
11 0 11
6 0 11_6
112 0 11_112
9 rows selected