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;