CREATE MATERIALIZED VIEW lesson_popularity_ranking AS 

SELECT 
	pl.id AS lesson_id,
	pl.title AS lesson_title,
	pc.id AS course_id,
	pc.title AS course_title,
	total_views_lesson.total_views AS total_views,
	table_unique_viewers.unique_v AS unique_viewers,
	ROUND(AVG(views_one_user_per_one_lesson.views_per_viewer), 2) AS avg_views_per_viewer, --также это можно было посчитать total_views/unique_viewers
	DENSE_RANK() OVER (
		ORDER BY total_views_lesson.total_views DESC NULLS LAST
	) AS popularity_rank,
	
	ROUND(table_unique_viewers.unique_v * 100.0 / table_enrolled_users.enrolled_to_course, 2) || '%' AS completion_rate
	
	FROM pet_lessons pl

	LEFT JOIN (
		SELECT 
			lesson_id,
			COUNT(*) AS total_views
		FROM pet_lesson_views
		GROUP BY lesson_id
	) AS total_views_lesson
		ON total_views_lesson.lesson_id = pl.id
	
	LEFT JOIN pet_courses pc
		ON pc.id = pl.course_id
	
	LEFT JOIN (
		SELECT
			lesson_id,
			COUNT(DISTINCT user_id) AS unique_v
		FROM pet_lesson_views plv
		GROUP BY plv.lesson_id
	) AS table_unique_viewers
		ON table_unique_viewers.lesson_id = pl.id
	
	LEFT JOIN (
		SELECT
			lesson_id,
			user_id,
			COUNT(*) AS views_per_viewer
		FROM pet_lesson_views plv
		GROUP BY user_id, lesson_id
		ORDER BY user_id, lesson_id
	) AS views_one_user_per_one_lesson
		ON views_one_user_per_one_lesson.lesson_id = pl.id
	
	LEFT JOIN (
		SELECT
			course_id,
			COUNT(DISTINCT user_id) AS enrolled_to_course
		FROM pet_enrollments pe
		GROUP BY course_id
	) AS table_enrolled_users
		ON table_enrolled_users.course_id = pc.id
	
		
		
	GROUP BY pl.id, pl.title, pc.id, pc.title, total_views_lesson.total_views, table_unique_viewers.unique_v, table_enrolled_users.enrolled_to_course
	ORDER BY lesson_id;