


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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