mysql每半小时平均值计算的sql语句

表结构：

create table `instance` (
 `id` int(11) unsigned not null auto_increment,
 `timestamp` int(10) unsigned default null,
 `cpu` decimal(8,3) default null,
 primary key (`id`)
) engine=innodb;

统计每半小时平均值，实现可以如下：

select `timestamp`-`timestamp`% (5*60) , avg(cup)
from `instance`
group by `timestamp`-`timestamp`% (5*60) 

另外还有：

select queuename, hour(dt) as h, floor(minute(dt) / 30) as v , 
   avg(memoryused), sum(memoryused), count(memoryused) 
from queue_resource 
where dt>='2014-04-14 00:00:00' 
group by h, v, queuename;