查詢與週期（甲骨文10 XE和11 XE）

Question

有路線圖（連接城市清單）：

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'); 

我需要的路徑：柏林=>羅馬=>新紐約=>達拉斯

變型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 

回程：=>羅馬=>倫敦

變體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：ERROR ORA-01436 CONNECT BY循環用戶數據

任何建議，如何獲得預期的結果與分層查詢？

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