如何获取每个月的第一行数据(postgres)

Question

我是postgres的初学者，想获得每个月的第一行(分组依据(，但我真的不确定如何获得。

我的表order如下：

order_id | cust_id | order_date
------------------------------------------------
order1   | cust1   | January 19, 2020, 1:00 PM
order2   | cust1   | January 30, 2020, 2:00 PM
order3   | cust1   | February 20, 2020, 3:00 PM
order4   | cust1   | February 28, 2020, 4:00 PM
order5   | cust2   | February 27, 2020, 4:00 PM

预期结果应符合：

order_id | cust_id | order_date
------------------------------------------------
order1   | cust1   | January 19, 2020, 1:00 PM
order3   | cust1   | February 20, 2020, 3:00 PM
order5   | cust2   | February 27, 2020, 4:00 PM

但我无法使用下面的查询获得上述结果，其中我获得的结果与表相同：

select distinct on (order_date)cust_id, order_date, order_id from order
group by delivery_date, customer_id, delivery_id
order by delivery_date asc

Answer 1

关闭。使用月份并取消group by:

select distinct on (cust_id, date_trunc('month', order_date) ) cust_id, order_date, order_id
from order
order by cust_id, date_trunc('month', order_date), delivery_date asc

Answer 2

您可以使用row_number分析函数，如下所示：

Select * from
(Select t.*,
row_number() over (partition by cust_id, date_trunc('month', order_date) 
order by order_date) as rn
From your_table t) t
Where rn = 1