I would 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

Things I need: the last user log entry of customers that's a particular status and it is a minimum of fifteen minutes old. When the user's latest log entry is really a certain status and it is fifteen minutes old, I have to take certain actions during my application.

How do you query this?

Presuming 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 might choose MAX(user_log_id) rather than `MAX(timestamp).

How about?

select * from user_log
where timestamp in (select max(timestamp) from user_log)
and status = 'certain'
and timestamp > DATE_SUB(now(), INTERVAL 15 MINUTE)

You must do "two queries" to obtain the last entry on the per user basis

Part one gets the final entry for every user, and the second reason is to obtain the data for your entry:

SELECT * FROM user_log u
INNER JOIN (
    SELECT MAX(user_log_id) as user_log_id, user_id
    FROM user_log
    WHERE TIMEDIFF(NOW(), timestamp) <= '00:15:00'
    GROUP BY user_id
) as a
ON u.user_log_id = a.user_log_id

This will highlight only customers which their last timestamp reaches least fifteen minutes old which status is @a_certain_status. If you wish to find customers their 15-minutes ago log had that status (disregarding any logs within the last 15 miutes, regardless of the status were in individuals logs), use @a1ex07's answer.

SELECT ul.*
FROM user_log AS ul
  JOIN
      ( SELECT user_id
             , MAX(`timestamp`) as maxts
        FROM user_log 
        GROUP BY user_id
        HAVING MAX(`timestamp`) <= NOW() - INTERVAL 15 MINUTE 
      ) AS ulg
    ON  ulg.user_id = ul.user_id 
    AND ulg.maxts = ul.`timestamp`
WHERE ul.status = @a_certain_status