mysql随机获取n条数据

作秒杀时，碰到从秒杀商品中随机取商品图的问题，能够用order by rand() limit *随机取出*条记录;

例如：

select gs.original_img from ecs_goods as gs where goods_id in( select goods_id from ecs_flash_sale where 1539323575 >= start_time and 1539323575 <= end_time ) order by rand() limit 1