https://practicum.yandex.ru/trainer/sql-data-analyst/lesson/1b657bc5-b116-424e-89eb-ebb059081b1f/task/7f0c0bf1-78fc-4c4e-bd10-bc90b9e48f18/


SELECT *
FROM
    (SELECT billing_country,
           AVG(total)
    FROM invoice
    WHERE EXTRACT(YEAR FROM CAST(invoice_date AS timestamp)) = 2009
        AND EXTRACT(MONTH FROM CAST(invoice_date AS timestamp)) IN (2, 5, 7, 10)
    GROUP BY billing_country) AS billing_country_avg