CREATE TABLE users (
    id INT PRIMARY KEY,
    name TEXT,
    age INT,
    email TEXT,
    registration_date DATE
);

INSERT INTO users VALUES
(1, 'Alice', 25, 'alice@mail.com', '2023-01-10'),
(2, 'Bob', 30, 'bob@gmail.com', '2023-02-05'),
(3, 'Charlie', 22, 'charlie@mail.com', '2023-02-20'),
(4, 'Diana', 28, 'diana@mail.com', '2023-03-01'),
(5, 'Ethan', 35, 'ethan@gmail.com', '2023-03-10');

SELECT * FROM users ORDER BY id;

CREATE TABLE courses (
    id INT PRIMARY KEY,
    title TEXT,
    category TEXT,
    created_at DATE
);

INSERT INTO courses VALUES
(1, 'SQL для начинающих', 'data', '2023-01-01'),
(2, 'Python для анализа данных', 'programming', '2023-01-15'),
(3, 'BI с нуля', 'business', '2023-03-01');
 

SELECT * FROM courses ORDER BY id;


CREATE TABLE lessons (
    id INT PRIMARY KEY,
    course_id INT,
    title TEXT,
    duration_min INT,
    FOREIGN KEY (course_id) REFERENCES courses(id)
);

INSERT INTO lessons VALUES
(1, 1, 'SELECT и FROM', 10),
(2, 1, 'JOIN', 15),
(3, 2, 'Pandas', 20),
(4, 3, 'Основы BI', 12),
(5, 3, 'Метрики и дашборды', 18);

SELECT * FROM lessons ORDER BY id;


CREATE TABLE enrollments (
    id INT PRIMARY KEY,
    user_id INT,
    course_id INT,
    enrolled_at DATE,
    FOREIGN KEY (user_id) REFERENCES users(id),
    FOREIGN KEY (course_id) REFERENCES courses(id)
);

INSERT INTO enrollments VALUES
(1, 1, 1, '2023-01-15'),
(2, 1, 2, '2023-02-01'),
(3, 2, 1, '2023-01-20'),
(4, 3, 2, '2023-03-05'),
(5, 4, 3, '2023-04-01');

SELECT * FROM enrollments ORDER BY id;

CREATE TABLE lesson_views (
    id INT PRIMARY KEY,
    user_id INT,
    lesson_id INT,
    viewed_at TIMESTAMP,
    FOREIGN KEY (user_id) REFERENCES users(id),
    FOREIGN KEY (lesson_id) REFERENCES lessons(id)
);

INSERT INTO lesson_views VALUES
(1, 1, 1, '2023-01-16 10:00:00'),
(2, 1, 2, '2023-01-16 10:15:00'),
(3, 2, 1, '2023-01-21 09:00:00'),
(4, 3, 3, '2023-03-06 12:00:00'),
(5, 4, 4, '2023-04-02 10:00:00'),
(6, 4, 5, '2023-04-03 10:00:00');

SELECT * FROM lesson_views ORDER BY id;

--Создаем materialized view c названием user_activity_summary

DROP MATERIALIZED VIEW IF EXISTS user_activity_summary;


CREATE MATERIALIZED VIEW user_activity_summary AS
WITH
enr AS (
  SELECT
    user_id,
    COUNT(*) AS enrollment_count
  FROM enrollments
  GROUP BY user_id
),
views AS (
  SELECT
    user_id,
    COUNT(DISTINCT lesson_id) AS viewed_lessons_count,
    MIN(viewed_at) AS first_view,
    MAX(viewed_at) AS last_view
  FROM lesson_views
  GROUP BY user_id
),
possible AS (
  SELECT
    e.user_id,
    COUNT(l.id) AS possible_lessons
  FROM enrollments e
  JOIN lessons l ON l.course_id = e.course_id
  GROUP BY e.user_id
)
SELECT
  u.id AS user_id,
  u.name,
  enr.enrollment_count,
  COALESCE(views.viewed_lessons_count, 0) AS viewed_lessons_count,
  views.last_view,
  views.first_view,
  ROUND(
    COALESCE(views.viewed_lessons_count, 0)::numeric
    / possible.possible_lessons * 100,
    2
  ) || '%' AS engagement_rate
FROM users u
JOIN enr ON enr.user_id = u.id
JOIN possible ON possible.user_id = u.id
LEFT JOIN views ON views.user_id = u.id
ORDER BY u.id;

--Проверка
SELECT * FROM user_activity_summary ORDER BY user_id;


--Создаем materialized view c названием course_summary

DROP MATERIALIZED VIEW IF EXISTS course_summary;


CREATE MATERIALIZED VIEW course_summary AS
WITH
-- 1) сколько уроков в каждом курсе
lessons_cnt AS (
  SELECT
    course_id,
    COUNT(*) AS lessons_count
  FROM lessons
  GROUP BY course_id
),

-- 2) сколько уникальных пользователей записано на курс
users_cnt AS (
  SELECT
    course_id,
    COUNT(DISTINCT user_id) AS unique_users
  FROM enrollments
  GROUP BY course_id
),

-- 3) просмотры по курсу (через lessons -> course_id)
views_cnt AS (
  SELECT
    l.course_id,
    COUNT(lv.id) AS total_views,
    MIN(lv.viewed_at) AS first_viewed_at,
    MAX(lv.viewed_at) AS last_viewed_at
  FROM lessons l
  LEFT JOIN lesson_views lv ON lv.lesson_id = l.id
  GROUP BY l.course_id
)

SELECT
  c.id AS course_id,
  c.title AS course_title,

  COALESCE(u.unique_users, 0) AS unique_users,
  COALESCE(ls.lessons_count, 0) AS lessons_count,
  COALESCE(v.total_views, 0) AS total_views,

  ROUND(
    COALESCE(v.total_views, 0)::numeric / NULLIF(COALESCE(u.unique_users, 0), 0),
    2
  ) AS avg_views_per_user,

  v.first_viewed_at,
  v.last_viewed_at
FROM courses c
LEFT JOIN users_cnt u ON u.course_id = c.id
LEFT JOIN lessons_cnt ls ON ls.course_id = c.id
LEFT JOIN views_cnt v ON v.course_id = c.id
ORDER BY c.id;

SELECT * FROM course_summary ORDER BY course_id;

--Создаем materialized view c названием platform_summary

DROP MATERIALIZED VIEW IF EXISTS platform_summary;

CREATE MATERIALIZED VIEW platform_summary AS
SELECT
  (SELECT COUNT(*) FROM users) AS total_users,
  (SELECT COUNT(*) FROM courses) AS total_courses,
  (SELECT COUNT(*) FROM lessons) AS total_lessons,

  (SELECT COUNT(DISTINCT user_id) FROM lesson_views) AS users_with_views,

  ROUND(
    (SELECT COUNT(*) FROM lessons)::numeric
    / (SELECT COUNT(*) FROM courses),
    2
  ) AS avg_lessons_per_course,

  ROUND(
    (SELECT COUNT(*) FROM lesson_views)::numeric
    / (SELECT COUNT(*) FROM lessons),
    2
  ) AS avg_views_per_lesson;
  
  SELECT * FROM platform_summary;
  
--Доп. задание: EXPLAIN
EXPLAIN SELECT * FROM user_activity_summary;
EXPLAIN SELECT * FROM course_summary;