SELECT parent_id AS parent_id, GROUP_CONCAT(DISTINCT child_id ORDER BY child_id) AS child_id_list
FROM `group_c`
group by parent_id
ORDER BY parent_id;
SELECT parent_id AS parent_id, GROUP_CONCAT(DISTINCT child_id ORDER BY child_id separator '|') AS child_id_list
FROM `group_c`
group by parent_id
ORDER BY parent_id;