Computing window functions for multiple dates - sql

Question

I have a table sales of consisting of user id's, products those users have purchased, and the date of purchase:
date
user_id
product
2021-01-01
1
apple
2021-01-02
1
orange
2021-01-02
2
apple
2021-01-02
3
apple
2021-01-03
3
orange
2021-01-04
4
apple
If I wanted to see product counts based on every users' most recent purchase, I would do something like this:
WITH latest_sales AS (
SELECT
date
, user_id
, product
, row_number() OVER(PARTITION BY user_id ORDER BY date DESC) AS rn
FROM
sales
)
SELECT
product
, count(1) AS count
FROM
latest_sales
WHERE
rn = 1
GROUP BY
product
Producing:
product
count
apple
2
orange
2
However, this will only produce results for my most recent date. If I looked at this on 2021-01-02. The results would be:
product
count
apple
2
orange
1
How could I code this so I could see counts of the most recent products purchased by user, but for multiple dates?
So the output would be something like this:
date
product
count
2021-01-01
apple
1
2021-01-01
orange
0
2021-01-02
apple
2
2021-01-02
orange
1
2021-01-03
apple
1
2021-01-03
orange
2
2021-01-04
apple
2
2021-01-04
orange
2
Appreciate any help on this.

Answer 1

I'm afraid the window function row_number() with the PARTITION BY user_id clause is not relevant in your case because it only focusses on the user_id of the current row whereas you want a consolidate view with all the users.
I dont have a better idea than doing a self-join on table sales :
WITH list AS (
SELECT DISTINCT ON (s2.date, user_id)
s2.date
, product
FROM sales AS s1
INNER JOIN (SELECT DISTINCT date FROM sales) AS s2
ON s1.date <= s2.date
ORDER BY s2.date, user_id, s1.date DESC
)
SELECT date, product, count(*)
FROM list
GROUP BY date, product
ORDER BY date
see the test result in dbfiddle