MySQL 8.0

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CREATE TABLE `tab` (
 `id` int(11) NOT NULL AUTO_INCREMENT,
 `val` int(11) DEFAULT NULL,
 `altval` int(11) DEFAULT NULL,
 PRIMARY KEY (`id`)
);
INSERT INTO tab(id, val, altval) VALUES(1,1, NULL);
INSERT INTO tab(id, val, altval) VALUES(2,2,3);
INSERT INTO tab(id, val, altval) VALUES(3,4,NULL);
INSERT INTO tab(id, val, altval) VALUES(4,5,6);
SELECT * FROM tab;


id
val
altval


1
1
null

2
2
3

3
4
null

4
5
6


WITH RECURSIVE cte AS (
   SELECT CAST(val AS CHAR(11)) val, CAST(altval AS CHAR(11)) AS altval, ROW_NUMBER() OVER(ORDER BY id) AS rn
   FROM tab
), rec AS (
  SELECT val AS result, rn
  FROM cte
  WHERE rn = 1
  UNION ALL
  SELECT altval AS result, rn
  FROM cte
  WHERE rn = 1
    AND altval IS NOT NULL
  UNION ALL
  SELECT CONCAT(rec.result, ',', cte.val), cte.rn
  FROM rec
  JOIN cte ON cte.rn = rec.rn+1
  UNION ALL
  SELECT CONCAT(rec.result, ',', cte.altval), cte.rn
  FROM rec
  JOIN cte ON cte.rn = rec.rn+1
  WHERE cte.altval IS NOT NULL
)
SELECT result
FROM rec
WHERE rn = (SELECT COUNT(*) FROM cte)


result


1,2,4,5

1,2,4,6

1,3,4,5

1,3,4,6


UPDATE tab SET altval = 9 WHERE id = 1;
SELECT * FROM tab;


id
val
altval


1
1
9

2
2
3

3
4
null

4
5
6


WITH RECURSIVE cte AS (
   SELECT CAST(val AS CHAR(11)) val, CAST(altval AS CHAR(11)) AS altval, ROW_NUMBER() OVER(ORDER BY id) AS rn
   FROM tab
), rec AS (
  SELECT val AS result, rn
  FROM cte
  WHERE rn = 1
  UNION ALL
  SELECT altval AS result, rn
  FROM cte
  WHERE rn = 1
    AND altval IS NOT NULL
  UNION ALL
  SELECT CONCAT(rec.result, ',', cte.val), cte.rn
  FROM rec
  JOIN cte ON cte.rn = rec.rn+1
  UNION ALL
  SELECT CONCAT(rec.result, ',', cte.altval), cte.rn
  FROM rec
  JOIN cte ON cte.rn = rec.rn+1
  WHERE cte.altval IS NOT NULL
)
SELECT result
FROM rec
WHERE rn = (SELECT COUNT(*) FROM cte)


result


1,2,4,5

9,2,4,5

9,2,4,6

1,2,4,6

9,3,4,5

1,3,4,5

1,3,4,6

9,3,4,6
id	val	altval
1	1	null
2	2	3
3	4	null
4	5	6