-- a)
SELECT title, pub_name FROM books;
-- b)
SELECT * FROM books ORDER BY pages_num DESC LIMIT 1;
-- c)
SELECT author
FROM (SELECT
        author,
        count(*)
      FROM books
      GROUP BY author) AS author_books
WHERE count > 5;
-- d)
SELECT *
FROM books
WHERE pages_num > 2 * (SELECT sum(pages_num)
                       FROM books) / (SELECT count(1)
                                      FROM books);