WITH t AS
(
SELECT 'a' fr_act, 'b' to_act FROM dual
UNION ALL SELECT 'b', 'c' FROM dual
UNION ALL SELECT 'c', 'd' FROM dual
UNION ALL SELECT 'ㄱ', 'ㄴ' FROM dual
UNION ALL SELECT 'ㄴ', 'ㄷ' FROM dual
)
SELECT fr_act
     , CONNECT_BY_ROOT(to_act) to_act
  FROM t
 WHERE CONNECT_BY_ISLEAF = 1
 CONNECT BY PRIOR fr_act = to_act
;

SELECT CONNECT_BY_ROOT(fr_act) fr_act
     , to_act
  FROM t a
 START WITH NOT EXISTS (SELECT 1 FROM t WHERE to_act = a.fr_act)
 CONNECT BY PRIOR to_act = fr_act
;