Question

我正在使用Mondial数据库架构并试图找到：对于每个国家/地区，找到人口最多且城市人口最多的城市。

现在我有：

SELECT Country.Name, city.name, MAX(city.population) Population
FROM city
Join Country
On Country.Code=City.Country
WHERE city.population IS NOT NULL
GROUP BY Country.Name, city.name
ORDER BY Country.Name;

这给了我所有的每个国家的城市及其人口，而不仅仅是最大的城市。

Answer 1

使用分析功能。这样的事情应该有效（未经测试）：

select
  country.name,
  city.name,
  city.population
from
  country
join
(
  select
    country,
    name,
    population,
    row_number() over ( partition by population desc) as rn
  from
    city
) city on
  city.country = country.code
  and city.rn = 1
order by
  country.name

Answer 2

在oracle中不知道，但如果在SQL Server中完成，可以这样做：

    Select * from
        (select 
        Country.Name,
        city.name,
        city.population,
        ROW_NUMBER() over(partition by Country.Name order by Country.Name,city.population desc) RowNum
    from Country inner join city city on Country.Code=City.Country) tbl
    where RowNum = 1

在oracle中类似于row_number的功能会有所帮助希望这有帮助。

Answer 3

这似乎有效。

根据包含聚合函数的列过滤查询结果也很有用。

SELECT ct.name AS "Country", c1.name AS "City", c1.population AS "Population"
FROM   city c1
    JOIN country ct
    ON c1.country = ct.code
WHERE  c1.population = (SELECT max(population)
                        FROM   city c2
                        WHERE  c1.country = c2.country)
ORDER BY country

Answer 4

您不能在多个选择中使用MAX 试试这个：

SELECT Country.Name, city.name, city.population
FROM city
Join Country
On Country.Code=City.Country
WHERE city.population IS NOT NULL and city.population in (SELECT MAX(population) FROM city limit 1)
GROUP BY Country.Name, city.name
ORDER BY Country.Name;

对于每个国家，找到人口最多的城市和城市人口

4 个答案: