在使用start with ... connect by 层次查询的用法报错：ORA-01436: CONNECT BY loop in user data 产生这个错误的原因，产生了死循环，解决办法有两种如下：

1、使用 connect by nocycle  （不推荐），该方法得不到正确的结果。
2、修改基础数据 。

借用网上有同学的提供的实验：

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;

上面两个sql执行的时候都会报ORA-01436: CONNECT BY loop in user data
解决方案入下：
方案1：在connect by  后面加上nocycle  ，不循环。得到的结果可能就不是你想要的了。
方案2：把第一条记录的parent_id 置为null ，这样做就不会产生死循环。

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