Find number of concurrent users in a SQL records

Question

I have the table of following structure:

UserID   StartedOn          EndedOn
1        2009-7-12T14:01    2009-7-12T15:01 
2        2009-7-12T14:30    2009-7-12T1         


        

Answer 1

This is NOT a solution. Since, at the time of this posting, the most upvoted solution has a really nasty CROSS JOIN for smaller numbers of rows and a really nasty TRIANGULAR JOIN for larger numbers of rows, I'd thought I'd post some code to make a more substantial amount of test data for people to do their testing with. Let the races begin. ;-)

DROP TABLE #Table
GO
WITH
cteStartedOn AS
(
 SELECT TOP 100000 --LOOK!  Change this number to vary the number of rows you're testing with.
        UserID = ABS(CHECKSUM(NEWID()))%1000,
        StartedOn = RAND(CHECKSUM(NEWID()))*DATEDIFF(dd,'2012','2013')+CAST('2012' AS DATETIME)
   FROM sys.all_columns ac1, sys.all_columns ac2
)
 SELECT UserID, StartedOn,
        EndedOn = DATEADD(ss,ABS(CHECKSUM(NEWID()))%36000,StartedOn) --10 hours max
   INTO #Table
   FROM cteStartedOn;