怎么用SQL统计每半小时内某个表的新增记录数

如何用SQL统计每半小时内某个表的新增记录数?
下面是每一个小时的语句。  
 SELECT 
  CONVERT(VARCHAR(10),createdate,120) AS 日期 ,DATEPART(hh,createdate) AS 小时,COUNT(*)AS 记录
  FROM transactionlog where CreateDate>[email protected]+' 00:00:00' and createdate<[email protected]+' 23:59:59'
  GROUP BY CONVERT(VARCHAR(10),createdate,120),DATEPART(hh,createdate) 
  order by 小时

那每半小时呢?

------解决方案--------------------

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SQL code
CONVERT(VARCHAR(10),createdate,120) AS 日期 ,
DATEPART(hh,createdate) AS 小时,COUNT(*)AS 记录
FROM transactionlog
where CreateDate>[email protected]+' 00:00:00' and createdate<[email protected]+' ……