Working with Subqueries

SELECT
    c.Name,
    cl.`Language`
FROM country c
INNER JOIN (
    SELECT CountryCode, `Language`
    FROM countrylanguage cl1
    WHERE
        Percentage = (
            SELECT MAX(Percentage)
            FROM countrylanguage cl2
            WHERE
                cl1.CountryCode = cl2.CountryCode
    )
) cl ON c.Code = cl.CountryCode


Name Language
Aruba Papiamento
Afghanistan Pashto
Angola Ovimbundu
Anguilla English
Albania Albaniana
Andorra Spanish
Netherlands Antilles Papiamento
United Arab Emirates Arabic
Argentina Spanish
Armenia Armenian
...

The subquery selects the most popular language for each country and returns a table with the country code and language name. Then this table is connected to the main one using the INNER JOIN.