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

I tried AlexKuznetsov's solution but the result was 49 :(

My solution:

/* Create temporary table and set all dates into 1 column,
so we can sort by this one column */
DECLARE @tmp table (
    Dates datetime,
    IsStartedDate bit )

INSERT INTO @tmp
    SELECT StartedOn, 1 FROM stats
    UNION ALL
    SELECT EndedOn, 0 FROM stats

DECLARE @currentlogins int, @highestlogins int, @IsStartedDate bit;
SET @currentlogins = 0;
SET @highestlogins = 0;

DECLARE tmp_cursor CURSOR FOR 
SELECT IsStartedDate FROM @tmp
ORDER BY Dates ASC

OPEN tmp_cursor

/* Step through every row, if it's a starteddate increment @currentlogins else decrement it
When @currentlogins is higher than @highestlogins set @highestlogins to the new highest value */
FETCH NEXT FROM tmp_cursor 
INTO @IsStartedDate

WHILE @@FETCH_STATUS = 0
BEGIN
    IF (@IsStartedDate = 1)
    BEGIN
        SET @currentlogins = @currentlogins + 1;
        IF (@currentlogins > @highestlogins)
            SET @highestlogins = @currentlogins;
    END
    ELSE
        SET @currentlogins = @currentlogins - 1;

    FETCH NEXT FROM tmp_cursor 
    INTO @IsStartedDate
END

CLOSE tmp_cursor
DEALLOCATE tmp_cursor

SELECT @highestlogins AS HighestLogins