Done

amaksatov/postgresql/OS-330.sql

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+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 (
+        CASE
+            WHEN COUNT(DISTINCT l.id) = 0 THEN NULL
+            ELSE COUNT(DISTINCT lv.lesson_id) * 1.0 / COUNT(DISTINCT l.id) * 100
+        END, 2)::TEXT || '%' 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
+HAVING COUNT(e.id) > 0;