declare @startDate datetime
declare @increment int
declare @perfCounter nvarchar(100)
declare @machineName nvarchar(100)
declare @startOfTheDay datetime
declare @numofpoints int


-- set @perfCounter = '% Processor Time'
set @perfCounter = 'Available Mbytes'
set @machineName = 'CONTOSO'

set @increment = 15
set @numofpoints = 100

set @startDate = DATEADD(minute, -1 * (@numofpoints + 1) * @increment,  GETDATE())

-- set @startOfTheDay = DATEFROMPARTS(YEAR(@startDate), MONTH(@startDate), DAY(@startDate))
set @startOfTheDay = convert(datetime, (convert(varchar, @startDate, 112)), 112)
print @startOfTheDay
set @startDate = DATEADD(minute, Floor(DATEDIFF(minute, @startOfTheDay, @startDate)/@increment)*@increment, @startOfTheDay)

SELECT top(@numofpoints)
      DATEADD(minute, Floor(DATEDIFF(minute, @startDate, pc.[LogTime])/@increment)*@increment, @startDate) LogTimeN
	  ,pc.MachineName
      ,pcd.Counter
      ,avg(pc.[Value]) Avg
      ,min(pc.[Value]) Min
      ,max(pc.[Value]) Max
  FROM [dbo].[PerformanceCounters] pc
  inner join [dbo].[PerformanceCountersDefinitions] pcd on pcd.id=pc.[CounterId]
  where pcd.Counter = @perfCounter
  and pc.logtime >= @startDate
  and pc.MachineName = @machineName
  and pcd.Machine = @machineName
  group by
      DATEADD(minute, Floor(DATEDIFF(minute, @startDate, pc.[LogTime])/@increment)*@increment, @startDate)
	  ,pc.MachineName
      ,pcd.Counter
  order by 1 desc