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    <p>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):</p> <pre><code>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 &gt;= StartedOn AND ChangeTime &lt; EndedOn GROUP BY ChangeTime ) AS ConcurrencyAtChangeTimes ------- 5 </code></pre> <p>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.</p> <p><strong>Edit:</strong> I was answering the OP's question: "how one could calculate this using T-SQL only". Note that the question does not mention performance.</p> <p>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:</p> <p>Consider the following alternatives</p> <ol> <li>Write a cursor </li> <li>Write a CLR cursor</li> <li>Write a loop on the client</li> <li>Use an RDBMS with decent cursors, such as Oracle or PostgreSql</li> <li>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.</li> </ol> <p>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.</p>
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    1. POFind number of concurrent users in a SQL records
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    1. COMAX() on a derived table, DISTINCT on a sub-query, GROUP BY. No offense but isn't that exactly what http://stackoverflow.com/questions/621884/database-development-mistakes-made-by-appdevelopers is about?
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    2. COUsing GROUP BY is what - a common mistake? Are you serious?
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    3. COThe point is that his query is highly inefficient.
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