Você realmente precisa arredondar o
TimeStamp
à hora. No SQL Server, isso é um pouco feio, mas fácil de fazer: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);