ORA-01436: 用户数据中的 CONNECT BY 循环

SELECT r1.region_id,parent_id
          FROM cnl_region r1
         WHERE r1.region_id =1         START WITH r1.region_id = 1
        CONNECT BY PRIOR r1.region_id = r1.parent_id

在运行上面的SQL语句时它会报ORA-01436的错误，报这个错误原因是因为产生了循环。后来查找我的数据发现，数据region_id为1的parent_id为24684,而region_id为24684的parent_id为1，这样就产生了循环后来我把数据改过来就运行正常了。

不改变数据将下面的SQL语句改为让它不产生循环

SELECT r1.region_id,parent_id
          FROM cnl_region r1
         WHERE r1.region_id =1         START WITH r1.region_id = 1
        CONNECT BY NOCYCLE PRIOR r1.region_id = r1.parent_id

SQL code 
--start with ... connect by 层次查询的用法
--建表语句
create table automobiles(
       part_id number(5)
         constraint pk_auto_part_id primary key,
       parent_id number(5)
         constraint fk_auto_ppid_ references automobiles(part_id),
       part_cname varchar2(30) not null,
       part_ename varchar2(30) not null,
       mp_cost number(9,2),
       desribe varchar2(20)     
);
--插入数据
insert into automobiles values( 1,1,'汽车','mobile',84321.99,'Assembly');
insert into automobiles values( 2,1,'车身','bodywork',19892.99,'Manufacture');
insert into automobiles values( 3,1,'发送机','engine',42128,'Purchase');
insert into automobiles values( 4,1,'附件','attached',15212,'Assembly');
insert into automobiles values( 5,2,'保险杠','bumper',4812.95,'Purchase');
insert into automobiles values( 6,2,'底盘','chassis',12795.11,'Manufacture');
insert into automobiles values( 7,2,'行李箱','Boot',812.11,'Manufacture');
insert into automobiles values( 8,6,'车轮','Wheel',2062.2,'Manufacture');
insert into automobiles values( 9,6,'挡泥板','Mudguard',990,'Manufacture');
insert into automobiles values( 10,8,'轮胎','Tyre',300,'Purchase');
insert into automobiles values( 11,3,'发送机盘','Bonnet',3212,'Manufacture');
insert into automobiles values( 12,3,'活塞','Piston',1112.2,'Manufacture');
insert into automobiles values( 13,3,'汽化器','Carburetter',712.29,'Manufacture');
insert into automobiles values( 14,4,'变速器','Gearbox',5712.25,'Manufacture');
insert into automobiles values( 15,4,'仪表板','Dashboard',538.92,'Manufacture');
insert into automobiles values( 16,14,'制动器','Carburetter',712.29,'Manufacture');
insert into automobiles values( 17,14,'变速杆','Gearshift',2001,'Manufacture');
insert into automobiles values( 18,17,'传动轴','Shaft',1101,'Manufacture');
insert into automobiles values( 19,15,'里程表','Milometer',350.28,'Purchase');
/
--分层sql脚本语句练习
select level,part_id,parent_id,part_cname,part_ename,mp_cost,desribe 
from automobiles
start with part_id=1
connect by prior part_id=parent_id 
order by level;
--缩进显示
select level,
    lpad(' ',2*level-1)||part_cname||' '||part_ename as partName
from automobiles
start with part_id=1
connect by prior part_id=parent_id 
order by level;
在执行select level语句的时候，报如下错误：ORA-01436: CONNECT BY loop in user data 解决方案：将第一条数据中的parent_id改为null，否则loop循环找parent_id就找不到了！

原文地址：http://blog.csdn.net/weikaifenglove/archive/2009/05/21/4206979.aspx