SELECT a.cd, a.nm
     , MAX(b.dt) dt
     , MAX(v) KEEP(DENSE_RANK LAST ORDER BY b.dt) v
  FROM a1 a
     , (SELECT cd, dt, v FROM b1
         UNION ALL
        SELECT cd, dt, v FROM b2
        ) b
 WHERE a.cd = b.cd
 GROUP BY a.cd, a.nm
 ORDER BY a.cd
;

SELECT a.cd, a.nm
     , b.dt, b.v
  FROM a1 a
     , (SELECT cd, dt, v
             , ROW_NUMBER() OVER(PARTITION BY cd ORDER BY dt DESC) rn
          FROM (SELECT cd, dt, v FROM b1
                 UNION ALL
                SELECT cd, dt, v FROM b2
                )
        ) b
 WHERE a.cd = b.cd
   AND b.rn = 1
 ORDER BY a.cd
;