Question

I might have oversimplified my problem here: How to use MAX in MySQL?

Given a user’s log table:

TABLE: user_log
- user_log_id (PK)
- user_id (FK)
- status
- timestamp

What I need: the very last user log entry of all users that is a certain status and is at least 15 minutes old. If the user’s latest log entry is a certain status and is 15 minutes old, I need to take certain actions in my application.

How do I query this?

Answer 1

Assuming timstamp is datetime type, and user_id,timestamp unique

SELECT a.*
FROM user_log a
INNER JOIN
(SELECT user_id, MAX(`timestamp`) as ts FROM 
 user_log b WHERE timestamp <= NOW()-interval 15 MINUTE 
 GROUP BY user_id)b
ON (b.user_id = a.user_id AND a.`timestamp` = b.ts)

If user_log_id is auto increment, you may select MAX(user_log_id) instead of `MAX(timestamp).