Select entry with maximum value of column after grouping

问题

I currently got this query:

SELECT 
    `users_sessions`.`userid` `users_sessions`.`lastActive` , `users`.`firstname`
FROM 
    `users_sessions` , `users`
WHERE 
    `users_sessions`.`lastActive` >= DATE_SUB( NOW( ) , INTERVAL 60 MINUTE )
AND 
    `users`.`uidNumber` = `users_sessions`.`userid`

It selects all sessions where lastActive is 1 hour ago maximum. Now, as users can have multiple sessions at the same time, I only want to select the one with the biggest lastActive value.

I know that I somehow got to use GROUP BY users_sessions.userid and then select the entry with the biggest lastActive.

How can I do this properly?

回答1:

SELECT 
    `users_sessions`.`userid`, `users_sessions`.`lastActive` , `users`.`firstname`
FROM 
    `users_sessions` , `users`
WHERE 
    `users_sessions`.`lastActive` >= DATE_SUB( NOW( ) , INTERVAL 60 MINUTE )
AND 
    `users`.`uidNumber` = `users_sessions`.`userid`
GROUP BY 
    `users_sessions`.`userid`, `users_sessions`.`lastActive` , `users`.`firstname`
HAVING
    `lastActive` = MAX(`lastActive`)

回答2:

Just select MAX(users_sessions.lastActive) instead of users_sessions.lastActive and use GROUP BY users.uidNumber. It should work.

来源：https://stackoverflow.com/questions/12873249/select-entry-with-maximum-value-of-column-after-grouping