SELECT a.billing_country,
    a.total_invoice,
    b.total_customer
FROM
(SELECT billing_country,
    COUNT(invoice_id) AS total_invoice
FROM
    (SELECT
        EXTRACT (YEAR FROM CAST (invoice_date AS date))  AS year,
        billing_country,
        invoice_id
    FROM invoice) AS i
RIGHT OUTER JOIN
    (SELECT
        EXTRACT (YEAR FROM CAST (invoice_date AS date))  AS year,
        SUM (total) AS sum
     FROM invoice
    WHERE EXTRACT (MONTH FROM CAST (invoice_date AS date)) IN (6,7,8)
    GROUP BY year
    ORDER BY sum DESC
    LIMIT 1) AS k ON i.year=k.year
GROUP BY billing_country) AS a
LEFT OUTER JOIN
(SELECT
    i.billing_country billing_country,
    COUNT(c.customer_id) total_customer
FROM invoice AS i
LEFT OUTER JOIN client AS c ON i.customer_id = c.customer_id
GROUP BY billing_country) as b ON a.billing_country = b.billing_country
ORDER BY total_invoice DESC,billing_country