SELECT a.billing_country AS country,
total_invoice,
total_customer

FROM (SELECT
billing_country, COUNT(total) AS total_invoice
FROM invoice
WHERE EXTRACT(YEAR FROM (CAST(invoice_date AS TIMESTAMP))) IN (SELECT year
FROM
(SELECT EXTRACT(YEAR FROM (CAST(invoice_date AS timestamp))) AS year, SUM(total) AS revenue
FROM invoice
WHERE EXTRACT(MONTH FROM (CAST(invoice_date AS timestamp))) IN (6,7,8) 
GROUP BY year
ORDER BY revenue DESC
LIMIT 1) AS correct_year)
GROUP BY billing_country
ORDER BY total_invoice DESC) AS s 

LEFT JOIN
(SELECT
billing_country, COUNT(customer_id) AS total_customer
FROM invoice
GROUP BY billing_country) AS a ON s.billing_country = a.billing_country
ORDER BY total_invoice DESC, country