added three data_mart_files

skonev/postgresql/dm_course_summary.sql

@@ -0,0 +1,39 @@
+BEGIN;
+
+CREATE MATERIALIZED VIEW course_summary AS 
+WITH courses_users AS (
+SELECT 
+	c.id AS course_id,
+	c.title AS course_name,
+	COUNT(DISTINCT user_id) AS unique_users
+FROM courses c
+LEFT join enrollments e ON c.id = e.course_id 
+GROUP BY c.id, c.title
+), 
+courses_views AS (
+    SELECT 
+        l.course_id, 
+        COUNT(DISTINCT l.id) AS lessons_count,
+        COUNT(lv.id) AS total_views,
+        MAX(lv.viewed_at) AS last_viewed_at,
+        MIN(lv.viewed_at) AS first_viewed_at
+    FROM lessons l
+    JOIN lesson_views lv ON l.id = lv.lesson_id 
+    GROUP BY l.course_id
+)
+SELECT 
+	cu.course_id,
+	cu.course_name,
+	cu.unique_users,
+	cv.lessons_count,
+	cv.total_views,
+	ROUND(cv.total_views::DECIMAL / NULLIF(cu.unique_users, 0), 2) AS avg_views_per_user,
+	cv.first_viewed_at,
+	cv.last_viewed_at
+FROM courses_users cu
+LEFT JOIN courses_views cv USING(course_id)
+ORDER BY cu.course_id;
+
+COMMIT;
+
+