Left outer join for multiple users - sql

Question

I have a database for rounds of golf. I want to see how many rounds each user has per month for the last year.
To do that I created this view last_12_months with this code
SELECT date_part('month'::text, dates.date) AS month,
date_part('year'::text, dates.date) AS year
FROM ( SELECT (generate_series((now() - '1 year'::interval), now(), '1 mon'::interval))::date AS date) dates;
month
year
1
2020
2
2020
and so on to ...
12
2020
The summary view for counting the rounds is very simple also rounds_count_by_users
user_id
month
year
count_all
1
1
2020
15
1
3
2020
12
1
5
2020
10
2
4
2020
7
2
8
2020
6
2
9
2020
3
Now for what I want, querying for each user with left outer join is quite simple with
select *
from last_12_months
left outer join rounds_count_by_users
on last_12_months.month = rounds_count_by_users.month
and last_12_months.year = rounds_count_by_users.year
and user_id = 1
Which gives me all the months even when the user has no played rounds. What I would like however is to be able to do this for every user and make a materialized view for easy querying. Is there a nice and easy way of doing this? To be clear this is the final table I want.
This query doesn't work at least, that much I know.
select *
from last_12_months
left outer join rounds_count_by_users
on last_12_months.month = rounds_count_by_users.month
and last_12_months.year = rounds_count_by_users.year
where user_id = 1
user_id
month
year
count_all
1
1
2020
15
1
2
2020
null
1
3
2020
12
1
4
2020
null
1
5
2020
10
1
6
2020
null
1
7
2020
null
1
8
2020
null
1
9
2020
null
1
10
2020
null
1
11
2020
null
1
12
2020
null
2
1
2020
null
2
2
2020
null
2
3
2020
null
2
4
2020
7
2
5
2020
null
2
6
2020
null
2
7
2020
null
2
8
2020
6
2
9
2020
3
2
10
2020
null
2
11
2020
null
2
12
2020
null
I made an SQL Fiddle for this (slightly different values but same schema)
PS: I know about table aliases and data modeling and that stuff. My question is strictly about how to achieve the final result.

Answer 1

This query:
select * from last_12_months
where (year = year(current_date) - 1 and month >= month(current_date))
or
(year = year(current_date) and month < month(current_date))
returns the rows of last_12_months for the last 12 months (not including the current month).
This query:
select distinct user_id from rounds_count_by_users
returns all the distinct user_ids (it would be better if these ids where stored in a users table).
You must CROSS join the above queries and then LEFT join rounds_count_by_users:
select u.user_id, m.month, m.year, r.count_all
from (
select * from last_12_months
where (year = year(current_date) - 1 and month >= month(current_date))
or
(year = year(current_date) and month < month(current_date))
) m cross join (select distinct user_id from rounds_count_by_users) u
left outer join rounds_count_by_users r
on m.month = r.month and m.year = r.year and u.user_id = r.user_id
order by u.user_id, m.month
See the demo.

Answer 2

So I managed it to do it this way after help in the comments.
This leaves me with exactly what I wanted.
with all_rows as (
select * from view_last_12_months inner join users on true
)
select id as user_id, all_rows.year, all_rows.month, count_all
from all_rows left outer join view_rounds_count_last_year last12
on last12.month = all_rows.month
and last12.year = all_rows.year
and all_rows.id = last12.user_id