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    2009-7-12T14:45
3        2009-7-12T14:47    2009-7-12T15:30
4        2009-7-12T13:01    2009-7-12T17:01
5        2009-7-12T14:15    2009-7-12T18:01
6        2009-7-12T11:01    2009-7-12T19:01
1        2009-7-12T16:07    2009-7-12T19:01

I need to find the maximal number of concurrent users that were on line. In the above table the result would be 5 because users set1={1,2,4,5,6} and set2={1,3,4,5,6} were online in the same period.

Do you have an idea how one could calculate this using T-SQL only?

Answer 1

Clearly the number of concurrent users only changes when a user either starts or ends a period, so it is enough to determine the number of concurrent users during starts and ends. So, reusing test data provided by Remus (thank you Remus):

DECLARE @Table TABLE 
(
  UserId int, 
  StartedOn datetime,
  EndedOn datetime
);

insert into @table (UserId, startedOn, EndedOn)
select 1, '2009-7-12 14:01', '2009-7-12 15:01'
union all select 2, '2009-7-12 14:30', '2009-7-12 14:45'
union all select 3, '2009-7-12 14:47', '2009-7-12 15:30'
union all select 4, '2009-7-12 13:01', '2009-7-12 17:01'
union all select 5, '2009-7-12 14:15', '2009-7-12 18:01'
union all select 6, '2009-7-12 11:01', '2009-7-12 19:01'
union all select 1, '2009-7-12 16:07', '2009-7-12 19:01';

SELECT MAX(ConcurrentUsers) FROM(
SELECT COUNT(*) AS ConcurrentUsers FROM @table AS Sessions 
JOIN 
(SELECT DISTINCT StartedOn AS ChangeTime FROM @table
) AS ChangeTimes
ON ChangeTime >= StartedOn AND ChangeTime < EndedOn 
GROUP BY ChangeTime
) AS ConcurrencyAtChangeTimes
-------
5

BTW using DISTINCT per se is not a mistake - only abusing DISTINCT is. DISTINCT is just a tool, using it in this context is perfectly correct.

Edit: I was answering the OP's question: "how one could calculate this using T-SQL only". Note that the question does not mention performance.

If the questions was this: "what is the fastest way to determine maximum concurrency if the data is stored in SQL Server", I would provide a different answer, something like this:

Consider the following alternatives

1. Write a cursor
2. Write a CLR cursor
3. Write a loop on the client
4. Use an RDBMS with decent cursors, such as Oracle or PostgreSql
5. For top performance, design your table differently, so that you can retrieve the answer in one index seek. This is what I do in my system if I need to deliver best possible performance.

If the question was "what is the fastest way to determine maximum concurrency using a T-SQL query", I would probably not answer at all. The reason: if I needed really good performance, I would not solve this problem in a T-SQL query.

Answer 2

You can order all events on date order and compute a running aggregate of current users logged in:

DECLARE @Table TABLE 
(
  UserId int, 
  StartedOn datetime,
  EndedOn datetime
);

insert into @table (UserId, startedOn, EndedOn)
select 1, '2009-7-12 14:01', '2009-7-12 15:01'
union all select 2, '2009-7-12 14:30', '2009-7-12 14:45'
union all select 3, '2009-7-12 14:47', '2009-7-12 15:30'
union all select 4, '2009-7-12 13:01', '2009-7-12 17:01'
union all select 5, '2009-7-12 14:15', '2009-7-12 18:01'
union all select 6, '2009-7-12 11:01', '2009-7-12 19:01'
union all select 1, '2009-7-12 16:07', '2009-7-12 19:01';

with cte_all_events as (
select StartedOn as Date
    , +1 as Users
    from @Table
union all 
select EndedOn as Date
    , -1 as Users
    from @Table),
cte_ordered_events as (
select Date
    , Users
    , row_number() over (order by Date asc) as EventId
    from cte_all_events)
, cte_agg_users as (
  select Date
    , Users
    , EventId
    , (select sum(Users) 
        from cte_ordered_events agg
        where agg.EventId <= e.EventId) as AggUsers
    from cte_ordered_events e)
select * from cte_agg_users


2009-07-12 11:01:00.000 1   1   1
2009-07-12 13:01:00.000 1   2   2
2009-07-12 14:01:00.000 1   3   3
2009-07-12 14:15:00.000 1   4   4
2009-07-12 14:30:00.000 1   5   5
2009-07-12 14:45:00.000 -1  6   4
2009-07-12 14:47:00.000 1   7   5
2009-07-12 15:01:00.000 -1  8   4
2009-07-12 15:30:00.000 -1  9   3
2009-07-12 16:07:00.000 1   10  4
2009-07-12 17:01:00.000 -1  11  3
2009-07-12 18:01:00.000 -1  12  2
2009-07-12 19:01:00.000 -1  13  1
2009-07-12 19:01:00.000 -1  14  0

Once you have this in place, finding the number of maximum concurrent sessions is trivial. As you see you have two moments when you had 5 users, at 14:30 (when user 2 logged in) and at 14:47 (when user 3 logged in). Just replace the last query that selects from the CTE to get the actual max:

select top(1) AggUsers 
    from cte_agg_users
    order by AggUsers desc

This solution uses CTEs so it will only work on SQL 2k5, if you're still on SQL 2000 you'll have to rewrite it using derived tables instead of CTEs.