15. Continents and Currencies

SELECT TableOne.ContinentCode, TableOne.CurrencyCode, TableOne.CurrencyUsage
FROM
(
    SELECT c.ContinentCode ,c.CurrencyCode, COUNT(*) AS 'CurrencyUsage'
    FROM Countries AS c
    WHERE c.CurrencyCode IS NOT NULL
    GROUP BY c.ContinentCode ,c.CurrencyCode
    HAVING COUNT(c.CurrencyCode) > 1
) AS TableOne
INNER JOIN
(SELECT currencies.ContinentCode, MAX(currencies.CurrencyUsage) AS MaxUsage
FROM
        (
        SELECT c.ContinentCode ,c.CurrencyCode, COUNT(*) AS 'CurrencyUsage'
        FROM Countries AS c
        WHERE c.CurrencyCode IS NOT NULL
        GROUP BY c.ContinentCode ,c.CurrencyCode
        HAVING COUNT(c.CurrencyCode) > 1
        ) AS currencies
GROUP BY currencies.ContinentCode
) AS TableTwo
ON TableOne.ContinentCode = TableTwo.ContinentCode AND TableOne.CurrencyUsage = TableTwo.MaxUsage
ORDER BY TableOne.ContinentCode