Split comma separated string into rows in mysql

Use a subquery of arbitrary digits to split your string.Instead of vals you can use '1,2,3'.

SELECT
  DISTINCT SUBSTRING_INDEX(SUBSTRING_INDEX(vals, ',', n.digit+1), ',', -1) val
FROM
  tt1
  INNER JOIN
  (SELECT 0 digit UNION ALL SELECT 1 UNION ALL SELECT 2 UNION ALL SELECT 3  UNION ALL SELECT 4 UNION ALL SELECT 5 UNION ALL SELECT 6) n
  ON LENGTH(REPLACE(vals, ',' , '')) <= LENGTH(vals)-n.digit;

See it working

For MySQL 8.0.4+

SELECT *
FROM
  JSON_TABLE(
          CONCAT('[', '1,2,3,4', ']'),
          "$[*]"
          COLUMNS(
              ids BIGINT(20) PATH "$"
              )
      ) AS tt

Concatenate square brackets ([]) around your string to make it into a JSON array. Then use JSON_TABLE to convert it into a table. See the MySQL JSON Table Functions for more info.