Added LEFT JOIN and NULLIF to Task 5

aharlamcev/postgresql/Pet-projectsql.sql

@@ -324,7 +324,7 @@ from CTE
 
 ------------------------------------------------------------------------------------------------------------------
 -- 5 Задание Популярность курсов
-create materialized view lesson_popularity_ranking AS
+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
@@ -338,24 +338,22 @@ 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,
+				1.00 * COUNT(lv.id) / nullif(COUNT(distinct lv.user_id), 0) 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
+				left 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,
+coalesce(vc.avg_views_per_user, 0),
 dense_rank() OVER (ORDER BY vc.total_views desc) as popularity_rank,
-ROUND(100.0 * unique_viewers / enrolled_to_course, 2) as completion_rate  
+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
+