Difference between revisions of "SELECT within SELECT Tutorial"

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(MAX on gdp is unnecessary when using ALL in this case. Both work, and MAX is cleaner, but this example is supposed to be all about the ALL.)
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<p class='imper'>Find the largest country in each continent, show the '''continent''', the '''name''' and the '''area''': </p>
<p class='imper'>Find the largest country (by area) in each continent, show the '''continent''', the '''name''' and the '''area''': </p>
<source lang='sql' class='def'>
<source lang='sql' class='def'>

Revision as of 21:03, 30 April 2013


This tutorial looks at how we can use SELECT statements within SELECT statements to perform more complex queries.

namecontinentarea populationgdp
AlbaniaEurope28748 2831741 12960000000
AlgeriaAfrica2381741 37100000 188681000000
AndorraEurope46878115 3712000000
AngolaAfrica1246700 20609294 100990000000

Using nested SELECT



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

world(name, continent, area, population, gdp)

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

List the name and continent of countries in the continents containing 'Belize', 'Belgium'.

SELECT name,continent
FROM world
WHERE continent IN (
  SELECT continent
  FROM world
  WHERE name IN ('Belize','Belgium'))

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

The per capita GDP is the gdp/population
SELECT name FROM world
  WHERE continent='Europe' AND gdp/population >
     (SELECT gdp/population FROM world
      WHERE name='United Kingdom')

Which country has a population that is more than Canada but less than Poland? Show the name and the population.

SELECT name,population FROM world
WHERE population BETWEEN
(SELECT population+1 FROM world WHERE name='Canada')
(SELECT population-1 FROM world WHERE name='Poland')

To get a well rounded view of the important features of SQL you should move on to the next tutorial concerning aggregates.

To gain an absurdly detailed view of one insignificant feature of the language, read on.

We can use the word ALL to allow >= or > or < or <=to act over a list.

Which countries have a GDP greater than any country in Europe? [Give the name only.]

SELECT name FROM world 
  WHERE gdp > ALL
   (SELECT gdp FROM world
    WHERE continent = 'Europe' 
      AND gdp IS NOT NULL)

We can refer to values in the outer SELECT within the inner SELECT. We can name the tables so that we can tell the difference between the inner and outer versions.

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

SELECT continent, name, population FROM world x
  WHERE population >= ALL
    (SELECT population FROM world y
        WHERE y.continent=x.continent
          AND population>0)
SELECT continent, name, area FROM world x
  WHERE area >= ALL
    (SELECT area FROM world y
        WHERE y.continent=x.continent)

The following questions are very difficult:

Find each country that belongs to a continent where all populations are less than 25000000. Show name, continent and population.

SELECT name,continent,population FROM world x
  WHERE 25000000 >= ALL (
    SELECT population FROM world y
     WHERE x.continent=y.continent
       AND y.population>0)

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

SELECT name, continent FROM world x WHERE
 population > ALL
 (SELECT population*3 FROM world y
 WHERE y.continent = x.continent
 AND y.name != x.name)
Clear your results