SQL (Structured Query Language) offers powerful capabilities for data retrieval and analysis. One such feature is the ability to use subqueries, also known as SELECT within SELECT statements. Subqueries allow for more complex and dynamic queries by nesting one SELECT statement inside another. In this article, we'll explore several examples of SQL subqueries and demonstrate how they can be used to solve advanced data analysis tasks.

List each country name where the population is larger than that of 'Russia'.

SELECT name FROM world
  WHERE population >
     (SELECT population FROM world
      WHERE name='Russia')

Show the countries in Europe with a per capita GDP greater than 'United Kingdom'.

SELECT name
FROM world
WHERE continent = 'Europe'
AND gdp/population > (
  SELECT gdp/population
  FROM world
  WHERE name = 'United Kingdom');

List the name and continent of countries in the continents containing either Argentina or Australia. Order by name of the country.

SELECT name,continent FROM world
WHERE continent IN 
(SELECT continent 
 FROM world 
 WHERE name IN('Australia','Argentina')
)

Which country has a population that is more than United Kingdom but less than Germany? Show the name and the population.

SELECT name,population FROM world
WHERE population > (SELECT population FROM world WHERE name='United Kingdom') AND population < (SELECT population FROM world WHERE name='Germany')

Germany (population 80 million) has the largest population of the countries in Europe. Austria (population 8.5 million) has 11% of the population of Germany.

Show the name and the population of each country in Europe. Show the population as a percentage of the population of Germany.

SELECT name, CONCAT(ROUND(population/
   (SELECT population FROM world WHERE name='Germany')
   *100),'%') FROM world WHERE continent='Europe'

Which countries have a GDP greater than every country in Europe? [Give the name only.] (Some countries may have NULL gdp values)

SELECT name FROM world 
   WHERE GDP > ALL(SELECT gdp FROM world WHERE continent='Europe')

Find the largest country (by area) in each continent, show the continent, the name and the area:

SELECT continent,name,area FROM world x
  WHERE population >= ALL
    (SELECT population FROM world y
        WHERE y.continent=x.continent)

List each continent and the name of the country that comes first alphabetically.

SELECT continent,name FROM world
   GROUP BY continent
   ORDER BY name,continent

Some countries have populations more than three times that of all of their neighbours (in the same continent). Give the countries and continents.

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

SQL subqueries provide a powerful mechanism for performing complex data analysis and manipulation tasks. By nesting SELECT statements within SELECT statements, we can construct dynamic queries that extract valuable insights from our databases. Whether comparing populations, analyzing economic indicators, or identifying relationships between data points, SQL subqueries offer a versatile toolset for data professionals and analysts.