SQL Select 来自 table 的最大值和最小值

Question

我正在使用 MySql。我有一个 table 是我从 table 个国家/地区创建的。 table 拉动大陆并计算 table 中的国家数。

Table 创建工作正常。然后我想拉动国家数量最多的大陆和国家数量最少的大陆。

create table cops as (select 
continent,
count(name) as number_of_countries
from country
group by continent);

select 
continent,
number_of_countries
from cops
where number_of_countries = (select MIN(number_of_countries)) OR (select MAX(number_of_countries));

我得到了整个 table:

continent   number_of_countries
Antarctica  5
South America   14
Oceania 28
North America   37
Europe  46
Asia    51
Africa  58

不过我想要的是：

continent   number_of_countries
Antarctica  5
Africa  58

抱歉，我不知道如何在此处制作 table，所以行很乱。

还有，有什么办法可以：

将查询合并为一个并获得所需的结果？
按国家数量对各大洲进行排名（有一个新的排名列，例如非洲将是 1，亚洲将是 2，等等）？

Answer 1

实现此目的的一种方法是使用 UNION，它可以让您结合来自多个查询的结果（前提是它们具有相同的列）。例如，

-- Get continent with greatest number of countries.
SELECT
    continent,
    number_of_countries
FROM cops
WHERE continent = (
    SELECT continent
    FROM cops
    ORDER BY number_of_countries DESC
    LIMIT 1
)

UNION

-- Get continent with least number of countries.
SELECT
    continent,
    number_of_countries
FROM cops
WHERE continent = (
    SELECT continent
    FROM cops
    ORDER BY number_of_countries ASC
    LIMIT 1
)

Answer 2

由于您已经有一个名为 cops 的 table，它包含每个大陆的国家数量，您可以这样做：

-- The UNION approach
select *
from cops
where number_of_countries = (select min(number_of_countries) from cops)
union
select *
from cops
where number_of_countries = (select max(number_of_countries) from cops);

或类似这样的内容：

select *
from cops
where number_of_countries in (
          (select min(number_of_countries) from cops),
          (select max(number_of_countries) from cops)
      );

关于你的第二个问题：使用用户变量：

select cops.*, @n := n + 1 as rank
from (select @n := 0) as init,
     cops
order by number_of_countries desc

Answer 3

您查询中的 WHERE 子句有误。用这样的东西替换它应该会给你你正在寻找的结果：

  where number_of_countries = ( SELECT MIN(number_of_countries) FROM cops )
     or number_of_countries = ( SELECT MAX(number_of_countries) FROM cops )

还有其他查询模式可以提供相同的结果。作为一种模式的示例，使用连接到内联视图：

  SELECT c.continent
       , c.number_of_countries
    FROM ( SELECT MIN(n.number_of_countries) AS min_noc
                , MAX(n.number_of_countries) AS max_noc
             FROM cops n
         ) m
    JOIN cops c
      ON c.number_of_countries IN (m.min_noc,m.max_noc)

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