MariaDB 10.4

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select version();


version()


10.4.26-MariaDB-1:10.4.26+maria~deb10-log


CREATE TABLE test(
  id INTEGER,
  reply_to_id INTEGER);

INSERT INTO test VALUES
  (12,NULL),
  (13,12),
  (14,12),
  (15,13);

Records: 4  Duplicates: 0  Warnings: 0


WITH RECURSIVE cte AS (
  SELECT 
     row_number() over (order by id) as x,
     id,
     reply_to_id 
  FROM test 
  WHERE reply_to_id=12
  
  UNION ALL
  
  SELECT x, test.id, test.reply_to_id
  FROM cte 
  INNER JOIN test on test.reply_to_id = cte.id
  )

SELECT * 
  FROM cte
  ORDER BY x,id;


x
id
reply_to_id


1
13
12

1
15
13

2
14
12