Count rows per hour in SQL Server with full date-time value as result

Question

How can I count the number of rows per hour in SQL Server with full date-time as result.

I've already tried this, but it returns only the hours

SELECT DATEPART(HOUR,TimeStamp), Count(*)
FROM [TEST].[dbo].[data]
GROUP BY DATEPART(HOUR,TimeStamp)
ORDER BY DATEPART(HOUR,TimeStamp)

Now the result is:

Hour   Occurrence 
----   ----------
10     2157
11     60740
12     66189
13     77096
14     90039

But I need this:

Timestamp               Occurrence
-------------------     ----------
2013-12-21 10:00:00     2157
2013-12-21 11:00:00     60740
2013-12-21 12:00:00     66189
2013-12-21 13:00:00     77096
2013-12-21 14:00:00     90039
2013-12-22 09:00:00     84838
2013-12-22 10:00:00     64238

Answer 1

You actually need to round the TimeStamp to the hour. In SQL Server, this is a bit ugly, but easy to do:

SELECT dateadd(hour, datediff(hour, 0, TimeStamp), 0) as TimeStampHour, Count(*)
FROM [TEST].[dbo].[data]
GROUP BY dateadd(hour, datediff(hour, 0, TimeStamp), 0)
ORDER BY dateadd(hour, datediff(hour, 0, TimeStamp), 0);