create table users (
    id serial primary key,
    name varchar(50) not null,
    age int not null,
    email varchar(100) not null,
    registration_date date not null
);

insert into users (name, age, email, registration_date) values
    ('Alice', 25, 'alice@mail.com', ; '2023-01-10'),
    ('Bob', 30, 'bob@gmail.com', '2023-02-05'),
    ('Charlie', 22, 'Charlie@mail.com', '2023-02-20'),
    ('Diana', 28, 'diana@mail.com', '2023-03-01'),
    ('Ethan', 35, 'ethan@gmail.com', '2023-03-10');

create table courses (
    id serial primary key,
    title varchar(100) not null,
    category varchar(30) not null,
    created_at date not_null
);

insert into courses (title, category, created_at) values
    ('SQL для начинающих', 'data', '2023-01-01'),
    ('Python для анализа данных', 'programming', '2023-01-15'),
    ('BI с нуля', 'business', '2023-03-01');

create table lessons (
    id serial primary key,
    course_id int references courses(id),
    title varchar(100) not null,
    duration_min int not null
);

insert into lessons (course_id, title, duration_min) values
    (1, 'SELECT и FROM', 10),
    (1, 'JOIN', 15),
    (2, 'Pandas', 20),
    (3, 'Основы BI', 12),
    (3, 'Метрики и дашборды', 18);

create table enrollments (
    id serial primary key,
    user_id int references users(id),
    course_id int references courses(id),
    enrolled_at date not null
);

insert into enrollments (user_id, course_id, enrolled_at) values
    (1, 1, '2023-01-15'),
    (1, 2, '2023-02-01'),
    (2, 1, '2023-01-20'),
    (3, 2, '2023-03-05'),
    (4, 3, '2023-04-01');

create table lesson_views (
    id serial primary key,
    user_id int references users(id),
    lesson_id int references lessons(id),
    viewed_at timestamp(0) not null
);

insert into lesson_views (user_id, lesson_id, viewed_at) values
    (1, 1, '2023-01-16 10:00:00'),
    (1, 2, '2023-01-16 10:15:00'),
    (2, 1, '2023-01-21 09:00:00'),
    (3, 3, '2023-03-06 12:00:00'),
    (4, 4, '2023-04-02 10:00:00'),
    (4, 5, '2023-04-03 10:00:00');