SELECT
ch.id_element,
ch.`table`,
ch.param,
ch.value_old,
(
SELECT GROUP_CONCAT(cu.name)
FROM c_user cu
WHERE FIND_IN_SET(cu.id, ch.value_old)
) AS value_new
FROM
c_history ch
LEFT JOIN
c_object co ON co.id = ch.id_element
WHERE
ch.`table` = 'c_object'
AND ch.param = 'ids_user_merge';