在PostgreSQL中查找過去n個月中的每個月的新記錄

Question

我有一個包含createddate的記錄表。我想返回過去6個日曆月中的每個月的新記錄，包括本月的部分月份。我很瞭解SQL Server，但不熟悉PostgreSQL。

我已經能夠與此查詢以獲取滾動個月的數據：從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，這將返回數據，4/22-5/21等
我想要的數據如下：6/1-6/21（部分本月），5/1-5/31等。

有什麼建議嗎？我也懷疑我可以在循環中做到這一點，但對語法還不夠熟悉。目前，我正在從PostgreSQL Maestro對備份文件進行測試。

謝謝。

Answer 1

我認爲date_trunc函數可能是你的朋友（見postgres docs）。你會做這樣的事我猜測：

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