SELECT m.fk_key
     , m.vlu
     , NVL(s1.sb1_vlu, s2.sb2_vlu) sb_vlu
  FROM master m
     , sub1 s1
     , sub2 s2
 WHERE m.fk_key = s1.sb1(+)
   AND m.fk_key = s2.sb2(+)
;

SELECT m.fk_key
     , m.vlu
     , NVL(s1.sb1_vlu, s2.sb2_vlu) sb_vlu
  FROM master m
  LEFT OUTER JOIN sub1 s1 ON m.fk_key = s1.sb1
  LEFT OUTER JOIN sub2 s2 ON m.fk_key = s2.sb2
;

마농님 감사합니다. 적용해 보겠습니다.~ ^^