B 테이블에 구분이 있어야 말이 되겠는데요? 구분이 없네요?

WITH t_a AS
(
SELECT '2020' yyyy, 1 seq FROM dual
UNION ALL SELECT '2020', 2 FROM dual
)
, t_b AS
(
SELECT '2020' yyyy, 1 seq, '변경전' gb, 100 amt FROM dual
UNION ALL SELECT '2020', 1, '변경후', 200 FROM dual
)
SELECT a.yyyy
     , a.seq
     , b.gb
     , NVL(b.amt, 0) amt
  FROM t_a a
  LEFT OUTER JOIN t_b b
  PARTITION BY (b.gb)
    ON a.yyyy = b.yyyy
   AND a.seq  = b.seq
 ORDER BY yyyy, seq, gb
;