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;
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