PET 5th task completed

aharlamcev/postgresql/Pet-projectsql.sql

@@ -322,5 +322,40 @@ avg_time_to_first_view
 from CTE
 ;
 
+------------------------------------------------------------------------------------------------------------------
+-- 5 Задание Популярность курсов
+create materialized view lesson_popularity_ranking AS
+with lessons_in_courses as (
+							select l.id as lesson_id, l.title as lesson_title, c.title as course_title,
+							COUNT(e.user_id) as enrolled_users
+							from lessons l
+							join courses c on l.course_id = c.id
+							join enrollments e on e.course_id = c.id
+							group by l.id, l.title, c.title
+							order by lesson_id
+							),
+views_cte as (
+				select lic.lesson_id,
+				COUNT(lv.id) as total_views,
+				COUNT(distinct lv.user_id) as unique_viewers,
+				1.00 * COUNT(lv.id) / COUNT(distinct lv.user_id) as avg_views_per_user,
+				COUNT(lic.enrolled_users) as enrolled_to_course 
+				from lessons_in_courses lic
+				join lesson_views lv on lic.lesson_id = lv.lesson_id
+				group by lic.lesson_id
+				)
+select lic.lesson_id, lic.lesson_title, lic.course_title, 
+vc.total_views, 
+vc.unique_viewers, 
+vc.avg_views_per_user,
+dense_rank() OVER (ORDER BY vc.total_views desc) as popularity_rank,
+ROUND(100.0 * unique_viewers / enrolled_to_course, 2) as completion_rate  
+from lessons_in_courses lic
+join views_cte vc on lic.lesson_id = vc.lesson_id
+;
+-- Единственный момент в том, что в итоговом MV нет уроков и курсов без просмотров, 
+--то есть тут только те уроки, у которых есть хотя бы 1 просмотр.
+-- Я не смог разобраться с NULL после LEFT JOIN таблиц и выдывало ошибку деления на 0 при подсчете completion_rate
+