SELECT EXTRACT(YEAR FROM (CAST(invoice_date AS timestamp))) AS year,
billing_country, COUNT(total) AS total_invoice
FROM invoice
GROUP BY year, billing_country
WHERE year 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)
ORDER BY total_invoice DESC