在group by子句postgresql中将null值返回为’0′

我使用count和group by来了解每天有多少订阅者购买：

select count(s.email) 
from subscriptions s,orders o 
where o.subcription_id=s.id 
group by s.created_at

结果：

created_at  count
------------------
04-04-2011  1
06-04-2011  2
07-04-2011  1
10-04-2011  5
11-04-2011  9

但是我仍然希望空行返回为’0′.我该怎么做呢？请注意,我必须使用这两个表.

created_at  count
------------------
04-04-2011  1
05-04-2011  0
06-04-2011  2
07-04-2011  1
08-04-2011  0
09-04-2011  0
10-04-2011  5
11-04-2011  9

解决方法

SELECT  s.created_at,COUNT(o.subsription_id)
FROM    subscriptions s
LEFT JOIN
        orders o
ON      o.subcription_id = s.id
GROUP BY
        s.created_at