SQL从表中选择MAX和MIN值

Question

我正在使用MySql。 我有一个从国家表格创建的表格。 表格将拉动大陆，并计算该表格中的国家/地区数量。

表创建工作正常。 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));

我得到整个桌子：

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

我只想要：

continent   number_of_countries
Antarctica  5
Africa  58

抱歉，我不知道如何在此处制作桌子，所以行很麻烦。

另外，有什么方法可以：

1. 将查询合并为一个并获得所需的结果？
2. 通过国家/地区对大陆进行排名（具有新的排名列，例如，非洲将为1，亚洲为2，等等）？

Answer 1

一种实现此目的的方法是使用UNION ，它可以让您合并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

由于您已经有了一个名为cops的表格，该表格可以保存每个大洲的国家数量，因此您可以执行以下操作：

-- 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);

或类似这样的东西：

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

关于第二个问题：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

您的查询中的WHERE子句是错误的。 用类似这样的东西代替应该给您想要的结果：

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

---

还有其他查询模式将给出相同的结果。 作为一种模式的示例，使用对内联视图的联接：

  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)