(SELECT
    t1.book_id, books_authors.author_name, t1.amountOfPurchase
FROM
    (SELECT
        user_books_buy.book_id AS id_book,
            COUNT(user_books_buy.book_id) as amountOfPurchase
    FROM
        user_books_buy
    GROUP BY user_books_buy.book_id) AS t1
        INNER JOIN
    books_authors ON t1.id_book = books_authors.id_book)


SELECT
    t2.authorName,
    t2.amountOfPurchase
FROM
    (SELECT
        books_authors.author_name AS authorName,
        MAX(t1.amountOfPurchase) AS amountOfPurchase
    FROM
        (SELECT
            user_books_buy.book_id AS id_book,
            COUNT(user_books_buy.book_id) AS amountOfPurchase
        FROM
            user_books_buy
        GROUP BY user_books_buy.book_id) AS t1
    INNER JOIN books_authors ON t1.id_book = books_authors.id_book
    GROUP BY books_authors.author_name) AS t2