SQL select one row for every n minutes - sql

Question

I'm using Microsoft SQL Server 2008, and have a data set that has entries for every few minutes, over a long period of time. I am using a program to graph the data, so i need to return about 20 values per hour. Some days the data is every minute, sometimes every five minutes, and sometimes every 8 or 9 minutes, so selecting every nth row won't give an even spread over time
eg for a sample in 2012, it looks like this :
DateTime
2012-01-01 08:00:10.000
2012-01-01 08:08:35.000
2012-01-01 08:17:01.000
2012-01-01 08:25:26.000
and for a sample the next year it looks like this:
DateTime
2013-07-20 08:00:00.000
2013-07-20 08:01:00.000
2013-07-20 08:02:00.000
2013-07-20 08:03:00.000
2013-07-20 08:04:00.000
at the moment I am using a statement like this:
SELECT * FROM [Master]
WHERE (((CAST(DATEPART(hour, DateTime)as varchar(2)))*60)
+CAST(DATEPART(minute, DateTime)as varchar(2))) % '5' = '0'
ORDER BY DateTime
This works fine for july 2013, but I miss most points in 2012, as it returns this
DateTime
2012-01-01 08:00:10.000
2012-01-01 08:25:26.000
2012-01-01 08:50:43.000
2012-01-01 09:15:59.000
2012-01-01 10:40:14.000
2012-01-01 11:05:30.000
What better way is there to do this?
EDIT: The table has a DateTime column, and a pressure column, and I need to output both and graph pressure against date and time.

Answer 1

Since they can be random for the hours, this should work for what you need:
Declare #NumberPerHour Int = 20
;With Cte As
(
Select DateTime, Row_Number() Over (Partition By DateDiff(Hour, 0, DateTime) Order By NewId()) RN
From Master
)
Select DateTime
From Cte
Where RN <= #NumberPerHour
Order By DateTime Asc
This will group the rows by the hour, and assign a random Row_Number ID to them, and only pull those with a Row_Number less than the number you're looking for per hour.