DROP MATERIALIZED VIEW IF EXISTS dmezentsev.course_summary;
CREATE MATERIALIZED VIEW dmezentsev.course_summary AS
WITH count_users AS (
	-- Кол-во уникальных пользователей в курсах
	SELECT
        c.id AS course_id,
		c.title AS course_title,
		COUNT(DISTINCT e.user_id) AS unique_users
	FROM dmezentsev.courses c 
	LEFT JOIN dmezentsev.enrollments e ON c.id = e.course_id
	GROUP BY c.id, c.title
),
	count_lessons AS (
	-- Кол-во уроков в курсах
	SELECT
        course_id,
		COUNT(id) AS lessons_count
	FROM dmezentsev.lessons 
	GROUP BY course_id
),
	views_count AS (
	-- Суммарное число просмотров по каждому курсу 
	SELECT 
        l.course_id,
		COUNT(lv.id) AS total_views,
		MIN(lv.viewed_at) AS first_viewed_at,
		MAX(lv.viewed_at) AS last_viewed_at
	FROM dmezentsev.lessons l 
	LEFT JOIN dmezentsev.lesson_views lv  ON l.id = lv.lesson_id
	GROUP BY l.course_id
)

SELECT
    cu.course_id,
	cu.course_title,
	cu.unique_users,
	cl.lessons_count,
	vc.total_views,
	ROUND(COALESCE(vc.total_views::numeric / 
		cu.unique_users, 0), 2) AS avg_views_per_user,
	vc.first_viewed_at,
	vc.last_viewed_at
FROM count_users cu 
LEFT JOIN count_lessons cl USING (course_id)
LEFT JOIN views_count vc USING (course_id)
ORDER BY cu.course_id;