SELECT b.countryname,
       DATE_FORMAT(a.issue_date, '%Y/%m') months,
       SUM(a.cases) case_num,
       SUM(a.new_vaccinations) vaccine_num
  FROM covid19_data a
 INNER JOIN covid19_country b
    ON a.countrycode = b.countrycode
 WHERE a.issue_date >= '2020-10-01'
   AND EXISTS (SELECT 1
                 FROM (SELECT countrycode,
                              SUM(new_vaccinations)
                         FROM covid19_data c
                        WHERE new_vaccinations > 0
                        GROUP BY 1
                        ORDER BY 2 DESC
                        LIMIT 10
                      ) c
                 WHERE a.countrycode = c.countrycode
               )
GROUP BY 1, 2;