在PostgreSQL中查找过去n个月中的每个月的新记录

Question

我有一个包含createddate的记录表。我想返回过去6个日历月中的每个月的新记录，包括本月的部分月份。我很了解SQL Server，但不熟悉PostgreSQL。

我已经能够与此查询以获取滚动个月的数据：从5/22-6/21

select 
COUNT(ID) as Total, 
COUNT(CASE WHEN createddate between (now() - '1 month'::interval)::timestamp AND now() THEN AG.ID END) as ThisMonth, 
COUNT(CASE WHEN createddate between (now() - '2 month'::interval)::timestamp AND (now() - '1 month'::interval)::timestamp THEN AG.ID END) as LastMonth, 
COUNT(CASE WHEN createddate between (now() - '3 month'::interval)::timestamp AND (now() - '2 month'::interval)::timestamp THEN AG.ID END) as PrevMonth, 
COUNT(CASE WHEN createddate between (now() - '4 month'::interval)::timestamp AND (now() - '3 month'::interval)::timestamp THEN AG.ID END) as PrevMonth2, 
COUNT(CASE WHEN createddate between (now() - '5 month'::interval)::timestamp AND (now() - '4 month'::interval)::timestamp THEN AG.ID END) as PrevMonth3, 
COUNT(CASE WHEN createddate between (now() - '6 month'::interval)::timestamp AND (now() - '5 month'::interval)::timestamp THEN AG.ID END) as PrevMonth4 
FROM a_group AG 

但在6/21，这将返回数据，4/22-5/21等
我想要的数据如下：6/1-6/21（部分本月），5/1-5/31等。

有什么建议吗？我也怀疑我可以在循环中做到这一点，但对语法还不够熟悉。目前，我正在从PostgreSQL Maestro对备份文件进行测试。

谢谢。

Answer 1

我认为date_trunc函数可能是你的朋友（见postgres docs）。你会做这样的事我猜测：

select 
COUNT(ID) as Total, 
COUNT(CASE WHEN createddate between date_trunc('month', now()) AND now() THEN AG.ID END) as ThisMonth, 
COUNT(CASE WHEN createddate between date_trunc('month', now()) - interval '1 month' AND date_trunc('month', now()) - interval '1 day' THEN AG.ID END) as LastMonth, 

等等