循环查询（Oracle 10 XE 和 11 XE）

Question

有路线图（连接城市列表）：

drop table aaa; 
create table aaa(a varchar2(10),  b varchar2(10));
insert into aaa values ('Rome','Berlin');
insert into aaa values ('London','Rome');
insert into aaa values ('London','New-York');
insert into aaa values ('New-York','Dallas');

我需要获取路径：柏林=>罗马=>纽约=>达拉斯

变体 1：

select sys_connect_by_path(DECODE(a, PRIOR a, b, a),'=>') PATH1
from aaa
start with a = 'Berlin' or b = 'Berlin'
connect by nocycle  Prior a = b or prior b = a

返回：=>罗马=>伦敦

变体 2：

select sys_connect_by_path(DECODE(a, PRIOR a, b, a),'=>') PATH1
from aaa
start with a = 'Berlin' or b = 'Berlin'
connect by Prior a = b or prior b = a

返回：错误 ORA-01436 CONNECT BY 在用户数据中循环

任何建议，如何通过分层查询获得预期结果？

Answer 1

select 
  sys_connect_by_path(b,'=>') PATH1
from 
(
  select 
    least(a, b) a, 
    greatest(a, b) b
  from aaa
)
start with a = 0 
connect by prior b = a

---

UPD：

select 
  sys_connect_by_path(b, '=>') PATH1
from 
  (
    select a, b from aaa
    union all
    select b, a from aaa
    union all
    select null, 'Berlin' from dual
  )
start with a is null
connect by nocycle prior b = a