SELECT data
     , lv
     , SUBSTRING_INDEX(SUBSTRING_INDEX(data, ',', lv), ',', -1) x
  FROM (SELECT '1,2,5' data) data_t
     , (SELECT 1 lv
        UNION ALL SELECT 2
        UNION ALL SELECT 3
        UNION ALL SELECT 4
        UNION ALL SELECT 5
        UNION ALL SELECT 6
        UNION ALL SELECT 7
        UNION ALL SELECT 8
        UNION ALL SELECT 9
        ) copy_t
 WHERE lv <= LENGTH(data) - LENGTH(REPLACE(data, ',', '')) + 1
;