I have the following two tables courses and enrollments in my PieCloudDB Database:

I now want to query the number of registrations for each course per month and the cumulative number of registrations for the course.

Below is my query:

SELECT
    EXTRACT(YEAR FROM e.enrollment_date) AS year,
    EXTRACT(MONTH FROM e.enrollment_date) AS month,
    c.course_name,
    COUNT(DISTINCT e.customer_id) AS monthly_registrations,
    SUM(COUNT(DISTINCT e.customer_id)) OVER (
        PARTITION BY e.course_id
        ORDER BY EXTRACT(YEAR FROM e.enrollment_date), EXTRACT(MONTH FROM e.enrollment_date)
    ) AS cumulative_registrations
FROM
    enrollments e
JOIN
    courses c ON e.course_id = c.course_id
GROUP BY
    e.course_id,
    c.course_name,
    year,
    month
ORDER BY
    year,
    month,
    c.course_name;

Then I got the results:

Now there is a problem. I want to get more complete information. When the number of registrations for a certain course in a certain month is 0, it also need to be outputted.

For example, for the above data I would like to get a result like this:

I don’t know how to do it, can anyone help me?(If you don’t know PieCloudDB, you can use PostgreSQL instead, thanks)