How to Retrieve the Latest Attendance for Each User Using MySQL\'s GROUP BY Clause?

Selecting the Latest Value Using Group By in MySQL

When attempting to retrieve the most recent attendance for each user using MySQL's GROUP BY clause, you may encounter instances where the initial attendance is returned instead of the desired latest value.

Using a Subquery

One solution is to utilize a subquery to identify the maximum timestamp for each member and join this subquery with the main table:

<code class="sql">SELECT *
FROM view_event_attendance AS t1
WHERE id_event = 782
AND timestamp = (SELECT MAX(timestamp)
                  FROM view_event_attendance AS t2 
                  WHERE t1.id_member = t2.id_member 
                    AND t1.id_event = t2.id_event 
                  GROUP BY id_event, id_member)
GROUP BY id_event, id_member;</code>

Using Concatenation and MAX()

An alternative approach involves concatenating the timestamp and attendance values and then using MAX() to select the combined values:

<code class="sql">SELECT substring(max(concat(from_unixtime(timestamp),attendance)) from 20) as attendance
FROM view_event_attendance
WHERE id_event = 782
GROUP BY id_event,id_member;</code>

This method returns only the latest attendance value for each member.

Selecting the Concatenated Timestamp/Attendance

If you prefer to retrieve the concatenated timestamp and attendance values, you can use the following query:

<code class="sql">SELECT concat(from_unixtime(timestamp),attendance), timestamp, attendance
FROM view_event_attendance
WHERE id_event = 782
GROUP BY id_event,id_member;</code>

Performance Considerations

When dealing with large datasets, subqueries can be computationally expensive. Concatenating values and using MAX() can provide better performance. However, it's important to evaluate the specific requirements and database characteristics to determine the optimal solution for your use case.

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