条件によるgroup byを行ってそれぞれ件数を表示

例）有効期限内と期限切れそれぞれの件数を１SQLで求めたい

mysql> select (expires_at < now()) expired, count(*) from token group by (expires_at < now());
+---------+----------+
| expired | count(*) |
+---------+----------+
|       0 |    57824 |　※期限内
|       1 |   271056 |　※期限切れ
+---------+----------+
2 rows in set (0.14 sec)