----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