added three data_mart_files

skonev/postgresql/dm_user_activity_summary.sql

@@ -0,0 +1,38 @@
+BEGIN;
+
+CREATE MATERIALIZED VIEW user_activity_summary AS 
+WITH users_views AS (
+SELECT
+	u.id AS user_id,
+	u.name, 
+	COUNT(DISTINCT lv.lesson_id) AS viewed_lessons_COUNT,
+	MAX(lv.viewed_at) AS last_view,
+	MIN(lv.viewed_at) AS first_view
+FROM users AS u
+JOIN lesson_views AS lv on u.id = lv.user_id
+GROUP BY u.id
+
+),
+users_engagement AS (
+SELECT 
+	e.user_id AS user_id,
+	COUNT(l.id) AS lessons_cnt,
+	COUNT(DISTINCT course_id) AS enrollment_count
+FROM enrollments AS e
+JOIN lessons AS l using(course_id)
+GROUP BY e.user_id
+)
+
+SELECT 
+	uv.user_id,
+	uv.name,
+	ue.enrollment_count,
+	uv.viewed_lessons_count,
+	uv.lASt_view,
+	uv.first_view,
+	ROUND(uv.viewed_lessons_count::DECIMAL / NULLIF(ue.lessons_cnt, 0) * 100, 2)  AS engagement_rate
+FROM users_views AS uv
+LEFT JOIN users_engagement AS ue using (user_id)
+ORDER BY uv.user_id
+
+COMMIT;