SELECT a.billing_country country,
    a.total_invoice total_invoice,
    b.total_customer
FROM
(SELECT billing_country ,
   COUNT(invoice_id) total_invoice
FROM invoice
WHERE EXTRACT (YEAR FROM CAST (invoice_date AS date)) in (SELECT
                                                                i.year
                                                          FROM
                                                         (SELECT
                                                               EXTRACT (YEAR FROM CAST (invoice_date AS date)) AS year,
                                                               SUM(total) 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 i)
                                                          GROUP BY billing_country) AS a
LEFT OUTER JOIN
(SELECT i.billing_country,
    COUNT(i.customer_id) total_customer
FROM client AS c
LEFT OUTER JOIN invoice AS i ON c.customer_id = i.customer_id
GROUP BY billing_country) AS b ON a.billing_country = b.billing_country
ORDER BY total_invoice DESC, country