最大日付のレコードのID取得
select t.autoId, m.takenDt, m.tag
from sales t
join (select tag, max(takenDt) as takenDt
from sales
group by tag) m
on t.tag = m.tag
and t.takenDt = m.takenDt;
MySQL Forums :: General :: select Max(field), field2 returns wrong field2
http://forums.mysql.com/read.php?20,371038