<div>
Would suggest an index on the startTime_f field. 
If that doesn't help, perhaps you can move over this table to SQL Server Express (free edition). 
Then Access is capable of executing Pass-Through queries really fast.
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kalics

<div>
First, create a query that lists the timeslots used: 
 

<pre><code class="code-1 nolinks" id="code-20-42664463-1">SELECT 
 UserLog.StartTime, 
 UserLog.EndTime
FROM 
 UserLog
GROUP BY 
 UserLog.StartTime, 
 UserLog.EndTime;
</code></pre>
Save this as TimeSlots. 
 
Then find the count of users having used any of these time slots: 
 

<pre><code class="code-10 nolinks" id="code-20-42664463-2">SELECT 
 TimeSlots.StartTime, 
 Count(*) AS Users
FROM 
 UserLog, 
 TimeSlots
WHERE 
 (TimeSlots.StartTime Between [UserLog]![StartTime] And [UserLog]![EndTime]) 
 OR 
 (TimeSlots.EndTime Between [UserLog]![StartTime] And [UserLog]![EndTime])
GROUP BY 
 TimeSlots.StartTime;
</code></pre>
Save this as TimeSlotUsers. 
 
Finally, find the maximum count of concurrent users: 
 

<pre><code class="code-1 nolinks" id="code-20-42664463-3">SELECT 
 DateValue([StartTime]) AS [Date], 
 Max(TimeSlotUsers.Users) AS MaxConcurrentUsers
FROM 
 TimeSlotUsers
GROUP BY DateValue([StartTime]);
</code></pre>
Of course, indexes on the time fields is a must.
</div>

<div>
Thanks very much Gustav. This works perfectly. Only one little question though - I have user session data for multiple applications on the same table. These queries work perfectly for concurrency for one application. 
 
When I did try to customise this for multiple applications, it comes up with the output of different applications listed, but it doesn't consider each application as unique &amp;&nbsp;does not calculate concurrency for that application, but considers only the time fields and therefore provides the concurrency for the overall database. 
 
Can you please help with any thoughts as to how to list max concurrency for each application? 
 
Much appreciated
</div>

<div>
Thanks very much Gustav. It worked like magic. Can't thank you enough.
</div>

<div>
Thank you so much. Incredibly helpful.
</div>

<div>
<div class="content wysiwyg-content">
Hi, I have an Access 2016 database which has a table with user sessions (session ID's, application name, session start date/time and session end date/time). I need to find out the maximum concurrency for each application for each date. 
 
I currently have a query as below which is taking too long (20 mins) to run but also comes out with only one day. Please can someone help with the algorithm for concurrency calculation &nbsp;for Access? 
 
SELECT TOP 1 Count(*) AS ['concurrency'], T1.Date 
FROM table AS T1, table AS T2 
WHERE (((T1.startTime_f) Between [T2].[startTime_f] And [T2].[endTIme_f])) 
GROUP BY T1.usersessionID, T1.Date, t2.date;
</div>
</div>

Hi, I have an Access 2016 database which has a table with user sessions (session ID's, application name, session start date/time and session end date/time). I need to find out the maximum concurrency for each application for each date. 

I currently have a query as below which is taking too long (20 mins) to run but also comes out with only one day. Please can someone help with the algorithm for concurrency calculation  for Access?

SELECT TOP 1 Count(*) AS ['concurrency'], T1.Date
FROM table AS T1, table AS T2
WHERE (((T1.startTime_f) Between [T2].[startTime_f] And [T2].[endTIme_f]))
GROUP BY T1.usersessionID, T1.Date, t2.date;

Calculate concurrent user sessions on Access DB

Microsoft Access is a rapid application development (RAD) relational database tool. Access can be used for both desktop and web-based applications, and uses VBA (Visual Basic for Applications) as its coding language.