Final fix

amaksatov/postgresql/OS-330.sql

@@ -1,15 +1,16 @@
-CREATE MATERIALIZED VIEW user_activity_summary AS
-SELECT 
-    u.id AS user_id,
-    u.name,
-    COUNT(DISTINCT e.course_id) AS enrollment_count,
-    COUNT(DISTINCT lv.lesson_id) AS viewed_lessons_count,
-    MAX(lv.viewed_at) AS last_viewed,
-    MIN(lv.viewed_at) AS first_viewed,
-    ROUND (COUNT(DISTINCT lv.lesson_id) * 1.0 / NULLIF(COUNT(DISTINCT l.id), 0) * 100, 2) AS "engagement_rate(%)"
-FROM amaksatov.users u
-LEFT JOIN enrollments e ON e.user_id = u.id
-LEFT JOIN lessons l ON l.course_id = e.course_id
-LEFT JOIN lesson_views lv ON lv.lesson_id = l.id AND u.id = lv.user_id
-GROUP BY u.id;
 
+CREATE MATERIALIZED VIEW user_activity_summary AS
+SELECT
+    u.id AS user_id, -- user_id
+    u.name, -- name
+    COUNT(DISTINCT e.course_id) AS enrollment_count, -- кол-во записей на курсы
+    COUNT(DISTINCT lv.lesson_id) AS viewed_lessons_count, -- кол-во просмотренных уроков
+    MAX(lv.viewed_at) AS last_viewed, -- последний просмотр
+    MIN(lv.viewed_at) AS first_viewed, -- первый просмотр
+    ROUND(COUNT(DISTINCT l.course_id) * 100.0 / COUNT(DISTINCT l.id), 2) || '%' AS engagement_rate -- вовлеченность(процент просмотренных/записанным)
+FROM amaksatov.users u -- основная таблица
+JOIN enrollments e ON e.user_id = u.id -- связь users с enrollments
+LEFT JOIN lesson_views lv ON lv.user_id = e.user_id -- связь lessons с lesson_views
+JOIN lessons l ON l.course_id = e.course_id  -- связь уроков с курсами пользователя
+GROUP BY u.id, u.name -- группировка по user_id и name
+ORDER BY user_id; -- сортировка по user_id