sql - Mondial DB 内部查询

Question

作为 Mondial DB 的一部分，我正在尝试回答以下问题。

-- For each country display the city with the highest population together with the number of the population.

这些表是：

国家（代码、名称、首都、省、地区、人口）

城市（名称，国家，省，经度，纬度，人口）

到目前为止，我有以下内容：

SELECT 
    Country.Name, MaxCity.CityName, MaxCity.Population
FROM
    (SELECT 
        MAX(Population) AS Population,
            City.Country,
            City.Name AS CityName
    FROM
        City
    GROUP BY City.Country) AS MaxCity
        JOIN
    Country ON Country.Code = MaxCity.Country;

这哪里出错了？

Answer 1

以这种方式尝试（我添加了一个额外的连接）

SELECT 
    Country.Name, City.CityName, City.Population
FROM
    Country Join City On Country.Code = City.Country
Join (SELECT 
        MAX(Population) AS Population,Country as Country from City Group By Country)  as X
On City.Country = x.Country and City.Population = x.Population

Answer 2

在源查询中需要按字段 city.name 添加到组中

    select country.name, maxcity.cityname, maxcity.population
    from
        (select 
            MAX(population) as population,
        city.country,
        city.name as cityname
    from city
    group by city.country, city.name) as maxcity
            join
    country on country.code = maxcity.country