WITH t AS
(
SELECT '2020' yyyy, 1 pk, 1000 amt
UNION ALL SELECT '2021', 1, 2000
UNION ALL SELECT '2023', 1, 4000
UNION ALL SELECT '2020', 2, 2222
UNION ALL SELECT '2022', 2, 3333
UNION ALL SELECT '2023', 2, 4444
)
SELECT a.yyyy
     , b.pk
     , IFNULL(c.amt, 0) amt
  FROM (WITH RECURSIVE y AS
       (SELECT '2020' yyyy
         UNION ALL
        SELECT yyyy + 1
          FROM y
         WHERE yyyy < '2023'
        )
       SELECT *
         FROM y
       ) a
 CROSS JOIN (SELECT DISTINCT pk FROM t) b
  LEFT OUTER JOIN t c
    ON a.yyyy = c.yyyy
   AND b.pk = c.pk
;