GROUP BY外移，提高SQL运行速度

数据表dh_order_detail 里一共有169247条数据

原始SQL

		SELECT
			FROM_UNIXTIME(order_time, "%H:%i") "time",
			city,
			district,
			LEFT (company, 6) company,
			goods_num,
			order_price,
			order_code,
			order_time
		FROM
			dh_order_detail
		WHERE
			province_id = 16
                GROUP BY
	                order_code
		ORDER BY
			order_time DESC
		LIMIT 50		SELECT