SampleTally, Sql Server

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declare @table table (SessionID int, UserName varchar(16), Started datetime, Ended datetime)
insert into @table
values
(100,'Test1','2015-07-26 00:03:05','2015-07-26 00:08:12')
,(101,'Test1','2015-07-26 00:04:05','2015-07-26 00:05:12')

declare @startDate datetime = (select min(cast(Started as date)) from @table)
declare @endDate datetime = (select dateadd(day,1,max(cast(Ended as date))) from @table)

;with NewTable as(
select 
	SessionID 
	,UserName
	,Started = CAST(DateAdd(minute, DateDiff(minute, 0, Started), 0) AS smalldatetime)
	,Ended = CAST(DateAdd(minute, DateDiff(minute, 0, Ended), 0) AS smalldatetime)
from @table
	),

E1(N) AS (select 1 from (values (1),(1),(1),(1),(1),(1),(1),(1),(1),(1))dt(n)),
    E2(N) AS (SELECT 1 FROM E1 a, E1 b), --10E+2 or 100 rows
    E4(N) AS (SELECT 1 FROM E2 a, E2 b), --10E+4 or 10,000 rows max
    cteTally(N) AS 
    (
        SELECT  TallyDate = dateadd(minute,ROW_NUMBER() OVER (ORDER BY (SELECT NULL)),@startDate) FROM E4
    )

--select
--	DateMinute = N
--	,SessionID
--	,Started
--	,Ended
--	,IsActive = case when (Started <=N and Ended >= N) then 1 else 0 end
--from NewTable t
--cross apply cteTally 
--where N <= (select max(Ended) from @table)
--order by SessionID, N

select
	DateMinute = N
	,ActiveSessions = sum(case when (Started <=N and Ended >= N) then 1 else 0 end)
from NewTable t
cross apply cteTally 
where N <= (select max(dateadd(minute,1,Ended)) from @table)
group by N
order by N

View schema
Execution time: 0,09 sec, rows selected: 11, rows affected: 2, absolute service time: 0,25 sec