SELECT d1.continent_code, d1.currency_code, d1.currency_usage FROM
	(SELECT `c`.`continent_code`, `c`.`currency_code`,
    COUNT(`c`.`currency_code`) AS `currency_usage` FROM countries as c
	GROUP BY c.currency_code, c.continent_code HAVING currency_usage > 1) as d1
LEFT JOIN
	(SELECT `c`.`continent_code`,`c`.`currency_code`,
    COUNT(`c`.`currency_code`) AS `currency_usage` FROM countries as c
	 GROUP BY c.currency_code, c.continent_code HAVING currency_usage > 1) as d2
ON d1.continent_code = d2.continent_code AND d2.currency_usage > d1.currency_usage

WHERE d2.currency_usage IS NULL
ORDER BY d1.continent_code, d1.currency_code;