Count unique rows GROUP(ed) BY different columns than used in DISTINCT ON

Question

I'm sure this has been asked over and over, but I can't quite find a simple example that I can fully grok.

I'm trying to deduplicate (do a DISTINCT ON ) by one column and COUNT records GROUPed By columns that differ from the column used to deduplicate, but without introducing subqueries.

Let's say I have a table with the following information:

order_num	date	region	timestamp_updated
001	2021-09-01	Murica	2021-09- 02 T19:00:01Z
001	2021-09-01	Murica	2021-09- 03 T19:00:01Z
002	2021-09-01	Yurop	2021-09-02T19:00:01Z
003	2021-09-01	Yurop	2021-09-03T19:00:01Z
004	2021-09-02	Yurop	2021-09-03T19:00:01Z

I would like to first get unique records with different order_num (keeping the most recently updated) ones AND then count groups or orders by date and region .

• De-duplicate (gets rid of the oldest order_num='001A' ):

  order_num	date	region	timestamp_updated
  001	2021-09-01	Murica	2021-09- 03 T19:00:01Z
  002	2021-09-01	Yurop	2021-09-02T19:00:01Z
  003	2021-09-01	Yurop	2021-09-03T19:00:01Z
  004	2021-09-02	Yurop	2021-09-03T19:00:01Z

• Then group by and count:

  date	region	count
  2021-09-01	Murica	1
  2021-09-01	Yurop	2
  2021-09-02	Yurop	1

I know how to do those two things separately ( distinct on(order_num) + order by timestamp_updated desc ) to deduplicate and then select count(*) + group by date, region ) And even together with subqueries. But I'd like to try to avoid subqueries as much as possible and here's where window functions (seem) to come in handy and I don't know much anything about those.

The closest thing I've been able to get are groups, but they show one record by each order_num . The records are correct, but they are duplicated:

select distinct on (order_num) date, region, count(1)over (
    partition by order_num
)
from orders_table
order by order_num, timestamp_updated desc;

That query ^^ shows:

date	region	count
2021-09-01	Murica	1	I think this is the first 001
2021-09-01	Murica	1	I think this is the second 001
2021-09-01	Yurop	2	I think this is the first Yurop: 002
2021-09-01	Yurop	2	I think this is the second Yurop: 003
2021-09-02	Yurop	1

Answer 1

You could get the max timestamp_updated per order_num, date, region and then aggregate again to get the counts per date, region using window function

select distinct 
       date, 
       region, 
       count(max(timestamp_updated)) over (partition by date, region) as counts 
from t
group by order_num, date, region;

DEMO