WITH t AS(
SELECT 1 id,'A'bsg,'0001'ftn FROM dual
UNION ALL 
SELECT 1,'B','0001' FROM dual
UNION ALL
SELECT 1,'C','0002' FROM dual
UNION ALL
SELECT 2,'A',null FROM dual
UNION ALL                       
SELECT 2,'B','0001' FROM dual
UNION ALL
SELECT 2,'C',NULL FROM dual
UNION ALL
SELECT 2,'D','0001' FROM dual
)
SELECT id,bsg,Decode(rn,1,ftn,null)ftn 
FROM
(
  select id,bsg,ftn,Row_Number() over (PARTITION BY id,ftn ORDER BY bsg)rn
  from t
  order by id,bsg
)

------解决方案--------------------
with temp1 as (select id,min(bsg) bsg,ftn from t1 group by id,ftnz)
select t1.id,t1.bsg,
case when  
exists (select 1 from temp1 where temp1.id=t1.id and temp1.bsg=t1.bsg) then t1.ftn
  else '' end ftn
from t1;