关于分时间段SQL查询语句求助
有一个实时数据库，时间和流量，想构造个查询每30分钟的平均值，请问如何实现啊


2012-5-20 0:00:003.441
2012-5-20 0:01:003.504
2012-5-20 0:02:007.636
2012-5-20 0:03:008.623
2012-5-20 0:04:004.313
...
2012-5-20 23:36:003.737
2012-5-20 23:37:006.258
2012-5-20 23:38:005.039
2012-5-20 23:39:004.064
2012-5-20 23:40:005.254

查询结果

2012-5-20 0:00:00XXX
2012-5-20 0:30:00XXX
2012-5-20 1:00:00XXX
2012-5-20 1:30:00XXX
...


[解决办法]

SQL code

SELECT 日期,AVG(数值) AS 数值FROM (    SELECT DATEADD(minute,DATEDIFF(minute,0,日期)/30*30,0) AS 日期,数值    FROM 表) AS AGROUP BY 日期