sql - 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)，但我不太确定如何。

我的表顺序如下:

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

预期的结果应该是:

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

但是我无法使用下面的查询获得上述结果，我得到的结果与表相同:

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。使用月份并去掉 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