GROUPBY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

改进，将GROUP BY外移

SELECT
	*
FROM
	(
		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
		ORDER BY
			order_time DESC
		LIMIT 50
	) o
GROUP BY
	order_code