group by
を追加する必要があります 条項
SELECT o.outfit_id, o.creation_date,
GROUP_CONCAT(i.manufacturer_product_id ORDER BY i.manufacturer_product_id) as manufacturer_product_ids,
GROUP_CONCAT(i.manufacturer_product_id ORDER BY i.manufacturer_product_id SEPARATOR ';') as manufacturer_product_ids_sep
FROM outfits o INNER JOIN
items_in_outfit io
ON o.outfit_id = io.outfit_id JOIN
items i
ON io.item_id = i.item_id JOIN
customer c
ON o.cus_id = c.cus_id
WHERE c.driver_id = 2463
group by o.outfit_id,o.creation_date
ORDER BY o.creation_date