MySQL MAX(datetime) not working - mysql

Question

I am trying to retrieve the max(date_entered) for a group of computer_ids.
The first query won't return accurate results. The second query gives me accurate results but essentially hangs unless I filter by a specific computer_id.
I'd rather use this first query
SELECT *, max(reports.date_entered)
FROM reports, hardware_reports
WHERE reports.report_id=hardware_reports.report_id
GROUP BY computer_id;
than this second query
SELECT *
FROM reports a
JOIN hardware_reports
ON a.report_id=hardware_reports.report_id
AND a.date_entered = (
SELECT MAX(date_entered)
FROM reports AS b
WHERE a.computer_id = b.computer_id)
and computer_id = 1648;
I need to either optimize second or get max to work in first.

Answer 1

You can alternative join it on a subquery that gets the latest record for every computer_ID.
SELECT a.*, c.*
FROM reports a
INNER JOIN
(
SELECT computer_ID, MAX(date_entered) date_entered
FROM reports
GROUP BY computer_ID
) b ON a.computer_ID = b.computer_ID
AND a.date_entered = b.date_entered
INNER JOIN hardware_reports c
ON a.report_id = c.report_id
To make it more efficient, provide an index on columns:
ALTER TABLE reports INDEX idx_report_compDate (computer_ID, date_entered)