为每一行构建层次结构，返回 pn 对于根行。
你可以这样做 connect by 使用 connect_by_root :

create table t (
  c1 varchar2(4), c2 varchar2(4)
);

insert into t values ( 'ABC1', 'ABC2' );
insert into t values ( 'ABC1', 'ABC9' );
insert into t values ( 'ABC2', 'ABC3' );
insert into t values ( 'ABC3', 'ABC4' );
insert into t values ( 'DEF1', 'DEF2' );
insert into t values ( 'GHI1', 'GHI2' );
insert into t values ( 'GHI2', 'GHI1' );
commit;

select * from (
  select distinct connect_by_root c1 rt, c2
  from   t
  connect by nocycle c1 = prior c2
)
where  rt <> c2
order  by rt, c2;

RT      C2     
ABC1    ABC2    
ABC1    ABC3    
ABC1    ABC4    
ABC1    ABC9    
ABC2    ABC3    
ABC2    ABC4    
ABC3    ABC4    
DEF1    DEF2    
GHI1    GHI2    
GHI2    GHI1

这个 nocycle 子句检测循环并停止处理。
或递归 with 通过在每次迭代中选择根的值：

with tree ( c1, c2, rt ) as (
  select c1, c2, c1 from t
  union all
  select t.c1, t.c2, tree.rt 
  from   tree
  join   t 
  on     t.c1 = tree.c2
) cycle c1 set is_cycle to 'Y' default 'N'
  select rt, c2 
  from   tree
  where  is_cycle = 'N'
  and    rt <> c2
  order  by rt, c2;

RT      C2     
ABC1    ABC2    
ABC1    ABC3    
ABC1    ABC4    
ABC1    ABC9    
ABC2    ABC3    
ABC2    ABC4    
ABC3    ABC4    
DEF1    DEF2    
GHI1    GHI2    
GHI2    GHI1

这个 cycle 子句检测循环。