SQL query to select all rows with max column value - mysql

Question

CREATE TABLE `user_activity` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`user_id` int(11) DEFAULT NULL,
`type` enum('request','response') DEFAULT NULL,
`data` longtext NOT NULL,
`created_at` datetime DEFAULT NULL,
`source` varchar(255) DEFAULT NULL,
`task_name` varchar(255) DEFAULT NULL,
PRIMARY KEY (`id`)
);
I have this data:-
Now I need to select all rows for user_id=527 where created_at value is the maximum. So I need the last 3 rows in this image.
I wrote this query:-
SELECT *
FROM user_activity
WHERE user_id = 527
AND source = 'E1'
AND task_name IN ( 'GetReportTask', 'StopMonitoringUserTask' )
AND created_at = (SELECT Max(created_at)
FROM user_activity
WHERE user_id = 527
AND source = 'E1'
AND task_name IN ( 'GetReportTask',
'StopMonitoringUserTask' ));
This is very inefficient because I am running the exact same query again as an inner query except that it disregards created_at. What's the right way to do this?

Answer 1

I would use a correlated subquery:
SELECT ua.*
FROM user_activity ua
WHERE ua.user_id = 527 AND source = 'E1' AND
ua.task_name IN ('GetReportTask', 'StopMonitoringUserTask' ) AND
ua.created_at = (SELECT MAX(ua2.created_at)
FROM user_activity ua2
WHERE ua2.user_id = ua.user_id AND
ua2.source = ua.source AND
ua2.task_name IN ( 'GetReportTask', 'StopMonitoringUserTask' )
);
Although this might seem inefficient, you can create an index on user_activity(user_id, source, task_name, created_at). With this index, the query should have decent performance.

Answer 2

Order by created_at desc and limit your query to return 1 row.
SELECT *
FROM user_activity
WHERE user_id = 527
AND source = 'E1'
AND task_name IN ( 'GetReportTask', 'StopMonitoringUserTask' )
ORDER BY created_at DESC
LIMIT 1;

Answer 3

I used EverSQL and applied my own changes to come up with this single-select query that uses self-join:-
SELECT *
FROM user_activity AS ua1
LEFT JOIN user_activity AS ua2
ON ua2.user_id = ua1.user_id
AND ua2.source = ua1.source
AND ua2.task_name IN ( 'GetReportTask', 'StopMonitoringUserTask' )
AND ua1.created_at < ua2.created_at
WHERE ua1.user_id = 527
AND ua1.source = 'E1'
AND ua1.task_name IN ( 'GetReportTask', 'StopMonitoringUserTask' )
AND ua2.created_at IS NULL;
However, I noticed that the response times of both queries were similar. I tried to use Explain to identify any performance differences; and from what I understood from its output, there are no noticeable differences because proper indexing is in place. So for readability and maintainability, I'll just use the nested query.