Difference between revisions of "SELECT within SELECT Tutorial"

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Line 36: Line 36:
 
   WHERE population >
 
   WHERE population >
 
     (SELECT population FROM world
 
     (SELECT population FROM world
       WHERE name='Russia')
+
       WHERE name='Romania')
 
</source>
 
</source>
  
Line 46: Line 46:
 
</source>
 
</source>
 
</div>
 
</div>
 +
 +
 
<div class='qu'>
 
<div class='qu'>
<p class='imper'>List the '''name''' and '''continent''' of countries in the continents containing 'Belize', 'Belgium'.</p>
+
<p class='imper'>Show the countries in Europe with a per capita GDP greater than 'United Kingdom'.</p>
 
+
<div class='hint' title='Per Capita GDP'>The per capita GDP is the gdp/population</div>
 
<source lang='sql' class='def'>
 
<source lang='sql' class='def'>
  
Line 54: Line 56:
  
 
<source lang='sql' class='ans'>
 
<source lang='sql' class='ans'>
SELECT name,continent
+
SELECT name FROM world
FROM world
+
  WHERE continent='Europe' AND gdp/population >
WHERE continent IN (
+
    (SELECT gdp/population FROM world
  SELECT continent
+
      WHERE name='United Kingdom')
  FROM world
+
  WHERE name IN ('Belize','Belgium'))
+
 
</source>
 
</source>
 
</div>
 
</div>
  
 
<div class='qu'>
 
<div class='qu'>
<p class='imper'>Show the countries in Europe with a per capita GDP greater than 'United Kingdom'.</p>
+
<p class='imper'>List the '''name''' and '''continent''' of countries in the continents containing 'Belize', 'Belgium'.</p>
<div class='hint' title='Per Capita GDP'>The per capita GDP is the gdp/population</div>
+
 
 
<source lang='sql' class='def'>
 
<source lang='sql' class='def'>
  
Line 71: Line 71:
  
 
<source lang='sql' class='ans'>
 
<source lang='sql' class='ans'>
SELECT name FROM world
+
SELECT name,continent
  WHERE continent='Europe' AND gdp/population >
+
FROM world
    (SELECT gdp/population FROM world
+
WHERE continent IN (
      WHERE name='United Kingdom')
+
  SELECT continent
 +
  FROM world
 +
  WHERE name IN ('Belize','Belgium'))
 
</source>
 
</source>
 
</div>
 
</div>
Line 80: Line 82:
  
 
<div class='qu'>
 
<div class='qu'>
<p class='imper'>Which country has a population that is more than Canada but less than Poland?</p>
+
<p class='imper'>Which country has a population that is more than Canada but less than Poland? Show the name and the population.</p>
  
 
<source lang='sql' class='def'></source>
 
<source lang='sql' class='def'></source>
Line 101: Line 103:
  
 
<div class='qu'>
 
<div class='qu'>
<p class='imper'>Which countries have a GDP greater than any country in Europe? [Give the '''name''' only.] </p>
+
<p class='imper'>Which countries have a GDP greater than every country in Europe? [Give the '''name''' only.] (Some countries may have NULL gdp values) </p>
  
 
<source lang='sql' class='def'>
 
<source lang='sql' class='def'>
Line 110: Line 112:
 
SELECT name FROM world  
 
SELECT name FROM world  
 
   WHERE gdp > ALL
 
   WHERE gdp > ALL
   (SELECT MAX(gdp) FROM world
+
   (SELECT gdp FROM world
 
     WHERE continent = 'Europe'  
 
     WHERE continent = 'Europe'  
 
       AND gdp IS NOT NULL)
 
       AND gdp IS NOT NULL)
Line 119: Line 121:
  
 
<div class='qu'>
 
<div class='qu'>
<p class='imper'>Find the largest country in each continent, show the '''continent''', the '''name''' and the '''population''': </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'>
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<source lang='sql' class='ans'>
 
<source lang='sql' class='ans'>
SELECT continent, name, population FROM world x
+
SELECT continent, name, area FROM world x
   WHERE population >= ALL
+
   WHERE area >= ALL
     (SELECT population FROM world y
+
     (SELECT area FROM world y
         WHERE y.continent=x.continent
+
         WHERE y.continent=x.continent)
          AND population>0)
+
 
</source>
 
</source>
 
</div>
 
</div>
  
  
The following questions are very difficult:
+
==Difficult Questions That Utilize Techniques Not Covered In Prior Sections==
 
+
 
<div class='qu'>
 
<div class='qu'>
 
<p class='imper'>Find each country that belongs to a continent where all populations are less than 25000000. Show '''name''', '''continent''' and '''population'''. </p>
 
<p class='imper'>Find each country that belongs to a continent where all populations are less than 25000000. Show '''name''', '''continent''' and '''population'''. </p>

Revision as of 19:19, 19 May 2014

SELECT within SELECT

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

namecontinentarea populationgdp
AfghanistanAsia6522302550010020343000000
AlbaniaEurope28748 2831741 12960000000
AlgeriaAfrica2381741 37100000 188681000000
AndorraEurope46878115 3712000000
AngolaAfrica1246700 20609294 100990000000
...

Using nested SELECT

Summary

Exercises

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='Romania')
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'.

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')

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'))


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')
AND
(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 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' 
      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)


Difficult Questions That Utilize Techniques Not Covered In Prior Sections

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

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