4 Nested SELECT Quiz-SQLZoo

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本文深入探讨了使用SQL进行复杂查询的方法，包括如何找出每个区域最小国家、属于人口全大于5万区域的国家、周边国家三分之一人口的国家等。通过具体示例，展示了子查询、聚合函数和比较运算符的巧妙运用。

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1. Select the code that shows the name, region and population of the smallest country in each region

 SELECT region, name, population FROM bbc x WHERE population <= ALL (SELECT population FROM bbc y WHERE y.region=x.region AND population>0)

2. Select the code that shows the countries belonging to regions with all populations over 50000

 SELECT name,region,population FROM bbc x WHERE 50000 < ALL (SELECT population FROM bbc y WHERE x.region=y.region AND y.population>0)

3. Select the code that shows the countries with a less than a third of the population of the countries around it

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

4. Select the result that would be obtained from the following code:

SELECT name FROM bbc
 WHERE population >
       (SELECT population
          FROM bbc
         WHERE name='United Kingdom')
   AND region IN
       (SELECT region
          FROM bbc
         WHERE name = 'United Kingdom')

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5. Select the code that would show the countries with a greater GDP than any country in Africa (some countries may have NULL gdp values)

SELECT name FROM bbc
 WHERE gdp > (SELECT MAX(gdp) FROM bbc WHERE region = 'Africa')

6. Select the code that shows the countries with population smaller than Russia but bigger than Denmark

SELECT name FROM bbc
 WHERE population < (SELECT population FROM bbc WHERE name='Russia')
   AND population > (SELECT population FROM bbc WHERE name='Denmark')

7. Select the result that would be obtained from the following code:

SELECT name FROM bbc
 WHERE population > ALL
       (SELECT MAX(population)
          FROM bbc
         WHERE region = 'Europe')
   AND region = 'South Asia'

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