sql子查询

Question

任何人都可以帮助我以下内容:

有些国家的人口是其邻国(在同一地区)的三倍以上.给国家和地区.

我的尝试:

select x.name, x.region
from bbc x
where x.population >all
(select population*3
from bbc y
where y.region = x.region)

语法是正确的但没有返回记录(应该返回3行)

查找属于所有人口少于25000000的地区的每个国家/地区.显示姓名,地区和人口.

我的尝试:

select name, region, population
from bbc
where region not in 
(select distinct region from bbc 
where population >= 25000000)

我用"不在".有没有办法使用"in"？

Answer 1

SELECT name, region 
FROM bbc x 
WHERE population/3 >= ALL
    (SELECT population
     FROM bbc y
     WHERE y.region=x.region
     AND x.name != y.name)

Answer 2

为了第一 :

你必须分工.第一步,找到一个国家的邻居.这必须是自动加入:

SELECT *
FROM bbc country
    INNER JOIN bbc neighbours
        ON country.region = neighbours.region
        AND country.name != neighbours.name

不要忘记将自己的国家排除在邻居之外!

其次,您可以计算一个国家的邻居拥有合适人口的数量:

sum(CASE WHEN country.population > neighbours.population * 3 THEN 1 ELSE 0 END)
(Group by country !)

与总数相比,你就完成了!

SELECT countryName 
FROM
(
    SELECT sum(CASE WHEN country.population > neighbours.population * 3 THEN 1 ELSE 0 END) as okNeighbours,
           count(*) as totalNeighbours
           country.name as countryName
    FROM bbc country
        INNER JOIN bbc neighbours
            ON country.region = neighbours.region
            AND country.name != neighbours.name
    GROUP BY country.name
)
WHERE totalNeighbours = okNeighbours

对于第二个:

SELECT name, region, population
FROM bbc
WHERE region IN (
    SELECT region
    FROM bbc
    GROUP BY region
    HAVING SUM(CASE WHEN population >= 25000000 THEN 1 ELSE 0 END) = 0
)

Answer 3

其他一些解决方案,为了兴趣而增加.

第一个查询:

SELECT name, 
       region 
FROM   bbc x 
WHERE  population > 
       -- this sub query finds each neighbour (not including itself) and returns the max populations multiplied by 3
       (SELECT 3 * MAX(population) 
        FROM   bbc y 
        WHERE  x.region = y.region 
               AND x.name <> y.name) 

第二个查询:

SELECT name, 
       region, 
       population 
FROM   bbc x 
WHERE  population < ALL 
       -- the ALL keyword allows comparison to be made against all the values in a list 
       -- this sub query finds each country that belongs to a region with populations less than 25 million and returns this as a list
       (SELECT population 
        FROM   bbc y 
        WHERE  y.region = x.region 
               AND population > 25000000) 

Answer 4

埃拉德，您的第一个答案几乎是正确的，只是缺少一个非常关键的组成部分：

SELECT x.name, x.continent
  FROM world x
  WHERE x.population >ALL(SELECT population*3
                            FROM world y
                            WHERE y.continent = x.continent
                            AND x.name<>y.name)

您会看到在执行子查询时要检查x.population> 3 *（同一大陆的y.populations），您必须指定不对同一国家/地区进行检查；否则，您将要检查x> 3x，这在数学上是不可能的。

Answer 5

SELECT name, region
FROM   bbc x
WHERE  population > 3 *
       (SELECT population 
        FROM bbc y 
        WHERE x.region=y.region
        ORDER BY population DESC limit 1,1)

您要照顾的人口是第二高人口价值的3倍，即限制1,1。第二个问题是缺少0，然后是正确的。

SELECT name, region, population 
FROM   bbc x
WHERE  (SELECT SUM(population) 
        FROM bbc y
        WHERE x.region=y.region) < 250000000