WITH t AS
(
SELECT 1 fr_cd, 2 to_cd FROM dual
UNION ALL SELECT 2, 4 FROM dual
UNION ALL SELECT 4, 8 FROM dual
UNION ALL SELECT 7, 8 FROM dual
UNION ALL SELECT 6, 4 FROM dual
UNION ALL SELECT 5, 6 FROM dual
UNION ALL SELECT 3, 5 FROM dual
)
SELECT gb
     , fr_cd, to_cd
     , LEVEL lv
     , SYS_CONNECT_BY_PATH(fr_cd, '-') ||'-'|| to_cd path
  FROM (SELECT 'A' gb, fr_cd, to_cd FROM t
         UNION ALL
        SELECT 'D' gb, to_cd, fr_cd FROM t
         WHERE fr_cd != 1
        )
 START WITH fr_cd = 1
 CONNECT BY NOCYCLE PRIOR to_cd = fr_cd
;