SELECT ps_orders.id_order,ps_order_detail.id_order, ps_orders.reference, ps_order_detail.product_quantity, ps_order_detail.product_reference
FROM ps_orders INNER JOIN ps_order_detail ON (ps_orders.id_order = ps_order_detail.id_order)

SELECT ps_orders.id_order,ps_order_detail.id_order, ps_orders.reference,
        GROUP_CONCAT(DISTINCT ps_order_detail.product_reference) AS product_reference2, 
        GROUP_CONCAT(DISTINCT ps_order_detail.product_quantity) AS product_quantity2 
FROM ps_orders INNER JOIN ps_order_detail ON (ps_orders.id_order = ps_order_detail.id_order)
GROUP BY ps_orders.reference;