SQL> ed
Wrote file afiedt.buf

  1  WITH zameni AS
  2    (SELECT '123456' base, '234567' zamena FROM dual UNION ALL
  3    SELECT '234567' base, '123456' zamena FROM dual UNION ALL
  4    SELECT 'ABC' base, 'DEF' zamena FROM dual UNION ALL
  5    SELECT 'ABC' base, 'GHK' zamena FROM dual UNION ALL
  6    SELECT 'DEF' base, 'ABC' zamena FROM dual UNION ALL
  7    SELECT 'DEF' base, 'GHK' zamena FROM dual UNION ALL
  8    SELECT 'GHK' base, 'ABC' zamena FROM dual UNION ALL
  9    SELECT 'GHK' base, 'DEF' zamena FROM dual)
 10  , Arts as (
 11    SELECT '123456' art FROM dual UNION ALL
 12    SELECT '234567' art FROM dual UNION ALL
 13    SELECT 'ABC' art FROM dual UNION ALL
 14    SELECT 'GHK' art FROM dual UNION ALL
 15    SELECT 'BEZZAMEN' art FROM dual
 16  )
 17  ,zz as (
 18  select art,nvl(base,art) base,nvl(zamena,art) zamena from arts a,zameni z where art=base(+))
 19  ,zzz as (
 20  select zz.*
 21   ,CONNECT_BY_ROOT base root
 22   from zz
 23  start with base=art
 24  connect by NOCYCLE prior zamena=base)
 25  select distinct max(zamena) minz --,max(zamena) max
 26  from zzz
 27* group by root
SQL> /

MINZ
--------
GHK
BEZZAMEN
234567