SQL query to count number of records within a given timeframe, filling in the gaps - tsql

Question

I am trying to count the number of records that fall between a given time period, generally 15 minutes, but I'd like to have this interval a variable. My table has a datetime column and I need to get a count of the number of records for every 15 minute interval between two dates and when there aren't any records for the 15-minute window, I need that time period with a zero. I've tried using CTE in different combinations and couldn't get it to work. I can generate the date series using a CTE, but haven't been able to get the actual data in the result. I have a working solution using a stored procedure with a WHILE loop. I was hoping to avoid this if possible in favor or a more elegant solution.
Here is my working loop using a temporary table:
declare #record_count int = 0
declare #end_date_per_query datetime
create table #output (
SessionIdTime datetime,
CallCount int)
while #date_from < #date_to
begin
set #end_date_per_query = DATEADD(minute, #interval, #date_from)
select #record_count = COUNT(*) from tbl WHERE SessionIdTime between #date_from and #end_date_per_query
insert into #output values (#date_from, #record_count)
set #date_from = #end_date_per_query
end
select * from #output order by sessionIdTime
drop table #output
Hopefully someone can help with a more elegant solution. Any help is appreciated.

Answer 1

A CTE works just fine:
-- Parameters.
declare #Start as DateTime = '20120901'
declare #End as DateTime = '20120902'
declare #Interval as Time = '01:00:00.00' -- One hour. Change to 15 minutes.
select #Start as [Start], #End as [End], #Interval as [Interval]
-- Sample data.
declare #Sessions as Table ( SessionId Int Identity, SessionStart DateTime )
insert into #Sessions ( SessionStart ) values
( '20120831 12:15:07' ), ( '20120831 21:51:18' ),
( '20120901 12:15:07' ), ( '20120901 21:51:18' ),
( '20120902 12:15:07' ), ( '20120902 21:51:18' )
select * from #Sessions
-- Summary.
; with SampleWindows as (
select #Start as WindowStart, #Start + #Interval as WindowEnd
union all
select SW.WindowStart + #Interval, SW.WindowEnd + #Interval
from SampleWindows as SW
where SW.WindowEnd < #End
)
select SW.WindowStart, Count( S.SessionStart ) as [Sessions]
from SampleWindows as SW left outer join
#Sessions as S on SW.WindowStart <= S.SessionStart and S.SessionStart < SW.WindowEnd
group by SW.WindowStart

Answer 2

Is this what you're looking for?
-- setup
DECLARE #interval INT, #start DATETIME
SELECT #interval = 15, #start = '1/1/2000 0:00:00'
DECLARE #t TABLE (id INT NOT NULL IDENTITY(1,1) PRIMARY KEY, d DATETIME)
INSERT INTO #t (d) VALUES
(DATEADD(mi, #interval * 0.00, #start)) -- in
,(DATEADD(mi, #interval * 0.75, #start)) -- in
,(DATEADD(mi, #interval * 1.50, #start)) -- out
-- query
DECLARE #result INT
SELECT #result = COUNT(*) FROM #t
WHERE d BETWEEN #start AND DATEADD(mi, #interval, #start)
-- result
PRINT #result