我使用count和group by获得每天注册的订阅者数量：
SELECT created_at,COUNT(email)
FROM subscriptions
GROUP BY created at;
结果：
created_at count ----------------- 04-04-2011 100 05-04-2011 50 06-04-2011 50 07-04-2011 300
我想要每天获得订阅者的累计总数。我如何得到这个？
created_at count ----------------- 04-04-2011 100 05-04-2011 150 06-04-2011 200 07-04-2011 500
