SELECT

      DISTINCT billing_country country,

      COUNT(DISTINCT invoice_id) total_invoice,

      total_customer

     

FROM invoice i

 

JOIN(SELECT country,

           COUNT(customer_id) total_customer

     FROM client

     GROUP BY country) AS customer_country ON customer_country.country=i.billing_country

 

WHERE EXTRACT(YEAR FROM CAST(invoice_date AS DATE)) =

            (SELECT year_number

             FROM (SELECT EXTRACT(YEAR FROM CAST(invoice_date AS DATE)) year_number,

                          EXTRACT(MONTH FROM CAST(invoice_date AS DATE)) month_number,

                          SUM(total) AS sum_total

                   FROM invoice

                   WHERE EXTRACT(MONTH FROM CAST(invoice_date AS DATE)) BETWEEN 6 AND 8

                   GROUP BY year_number, month_number) AS yms

             

            GROUP BY year_number

            ORDER BY SUM(sum_total) DESC

            LIMIT 1)

           

GROUP BY billing_country, total_customer

ORDER BY total_invoice DESC, country ASC