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

A naive approach:
You can test if another user b is currently logged in when user a logs in with

a.StartedOn BETWEEN b.StartedOn AND b.EndedOn

And someone has to be the "final logon" to the set of "the most concurrent users".
If you now go through all records (as a) and check how many other users (b) where logged in at the time and then order the list (desc) the first result is the maximum number of concurrent users.

SELECT
  a.id, a.UserId, a.StartedOn, a.EndedOn,  
  (  
    SELECT    
      Count(*)      
    FROM    
      logons as b      
    WHERE    
      a.StartedOn BETWEEN b.StartedOn AND b.EndedOn            
  ) as c
FROM
  logons as a 
ORDER BY
  c desc

And now read Database development mistakes made by application developers to see how inefficient (or even wrong) this is ;-)
e.g. you have a large temporary table that the order by operates on without any index to help the sql server.

(and btw: I tested this with MySQL because I don't have a sql server at hand right now)