seg0ro,
SELECT `group`.*, MAX(`elements`.`time`) AS `mtime` FROM `group` JOIN `elements`
USING(`mainid`)
GROUP BY `group`.`mainid`
ORDER BY `mtime` DESC seg0ro,
SELECT `group`.*, MAX(`elements`.`time`) AS `mtime` FROM `group` JOIN `elements`
USING(`mainid`)
GROUP BY `group`.`mainid`
ORDER BY `mtime` DESC