Difference between revisions of "SELECT within SELECT Tutorial"

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==SELECT within SELECT==
 
==SELECT within SELECT==
 
This tutorial looks at how we can use SELECT statements within SELECT statements to perform more complex queries.
 
This tutorial looks at how we can use SELECT statements within SELECT statements to perform more complex queries.
 +
<div class="ref_section">
 +
<table class='db_ref'>
 +
<tr><th>name</th><th>continent</th><th>area</th>
 +
<th>population</th><th>gdp</th></tr>
 +
<tr><td>Afghanistan</td><td>Asia</td><td class="r">652230</td><td class="r">25500100</td><td class="r">20343000000</td></tr>
 +
<tr><td>Albania</td><td>Europe</td><td class="r">28748 </td><td class="r">2831741 </td><td class="r">12960000000 </td></tr>
 +
<tr><td>Algeria</td><td>Africa</td><td class="r">2381741 </td><td class="r">37100000 </td><td class="r">188681000000 </td></tr>
 +
<tr><td>Andorra</td><td>Europe</td><td class="r">468</td><td class="r">78115 </td><td class="r">3712000000 </td></tr><tr>
 +
<td>Angola</td><td>Africa</td><td class="r">1246700 </td><td class="r">20609294 </td><td class="r">100990000000 </td></tr>
 +
<tr>
 +
<td colspan='5'>...</td>
 +
</tr>
 +
</table>
 +
</div>
  
 
[[Using_nested_SELECT |Using nested SELECT]]
 
[[Using_nested_SELECT |Using nested SELECT]]
 +
 +
<div class="progress_panel"><div>
 +
  <div class="summary">Summary</div>
 +
  <div class="progressbarbg">
 +
    <div class="progressbar"></div>
 +
  </div>
 +
</div></div>
 +
 
==Exercises==
 
==Exercises==
  
 
<div class='qu'>
 
<div class='qu'>
 
<p class='imper'>List each country '''name''' where the '''population''' is larger than 'Russia'. </p>
 
<p class='imper'>List each country '''name''' where the '''population''' is larger than 'Russia'. </p>
−
<pre space="preserve">bbc(name, region, area, population, gdp)</pre>
+
<pre space="preserve">world(name, continent, area, population, gdp)</pre>
  
  
 
<source lang='sql' class='def'>
 
<source lang='sql' class='def'>
−
SELECT name FROM bbc
+
SELECT name FROM world
 
   WHERE population >
 
   WHERE population >
−
     (SELECT population FROM bbc
+
     (SELECT population FROM world
 
       WHERE name='Russia')
 
       WHERE name='Russia')
 
</source>
 
</source>
  
 
<source lang='sql' class='ans'>
 
<source lang='sql' class='ans'>
−
SELECT name FROM bbc
+
SELECT name FROM world
 
   WHERE population >
 
   WHERE population >
−
     (SELECT population FROM bbc
+
     (SELECT population FROM world
 
       WHERE name='Russia')
 
       WHERE name='Russia')
 
</source>
 
</source>
 
</div>
 
</div>
−
 
 
<div class='qu'>
 
<div class='qu'>
−
<p class='imper'>List the '''name''' and '''region''' of countries in the regions containing 'India', 'Iran'.</p>
+
<p class='imper'>List the '''name''' and '''continent''' of countries in the continents containing 'Belize', 'Belgium'.</p>
  
 
<source lang='sql' class='def'>
 
<source lang='sql' class='def'>
Line 33: Line 54:
  
 
<source lang='sql' class='ans'>
 
<source lang='sql' class='ans'>
−
SELECT name,region FROM bbc
+
SELECT name,continent
−
  WHERE region IN
+
FROM world
−
    (SELECT region FROM bbc
+
WHERE continent IN (
−
    WHERE name IN ('India','Iran'))
+
  SELECT continent
 +
  FROM world
 +
  WHERE name IN ('Belize','Belgium'))
 
</source>
 
</source>
 
</div>
 
</div>
−
 
  
 
<div class='qu'>
 
<div class='qu'>
Line 49: Line 71:
  
 
<source lang='sql' class='ans'>
 
<source lang='sql' class='ans'>
−
SELECT name FROM bbc
+
SELECT name FROM world
−
   WHERE region='Europe' AND gdp/population >
+
   WHERE continent='Europe' AND gdp/population >
−
     (SELECT gdp/population FROM bbc
+
     (SELECT gdp/population FROM world
 
       WHERE name='United Kingdom')
 
       WHERE name='United Kingdom')
 
</source>
 
</source>
Line 65: Line 87:
  
 
<source lang='sql' class='ans'>
 
<source lang='sql' class='ans'>
−
SELECT name FROM bbc WHERE
+
SELECT name FROM world WHERE
 
  population >
 
  population >
 
  (SELECT population
 
  (SELECT population
−
   FROM bbc WHERE name='Canada')  
+
   FROM world WHERE name='Canada')  
 
     AND
 
     AND
 
   population <
 
   population <
 
  (SELECT population
 
  (SELECT population
−
   FROM bbc WHERE name='Algeria')
+
   FROM world WHERE name='Algeria')
 
</source>
 
</source>
 
</div>
 
</div>
  
  
−
[http://sqlzoo.net/w/index.php/SUM_and_COUNT To get a well rounded view of the important features of SQL you should move on to the next tutorial concerning aggregates.]
+
[[SUM_and_COUNT |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.
 
To gain an absurdly detailed view of one insignificant feature of the language, read on.
Line 91: Line 113:
  
 
<source lang='sql' class='ans'>
 
<source lang='sql' class='ans'>
−
SELECT name FROM bbc
+
SELECT name FROM world
 
   WHERE gdp > ALL
 
   WHERE gdp > ALL
−
   (SELECT MAX(gdp) FROM bbc
+
   (SELECT MAX(gdp) FROM world
−
     WHERE region = 'Europe'  
+
     WHERE continent = 'Europe'  
 
       AND gdp IS NOT NULL)
 
       AND gdp IS NOT NULL)
 
</source>
 
</source>
Line 102: Line 124:
  
 
<div class='qu'>
 
<div class='qu'>
−
<p class='imper'>Find the largest country in each region, show the '''region''', the '''name''' and the '''population''': </p>
+
<p class='imper'>Find the largest country in each continent, show the '''continent''', the '''name''' and the '''population''': </p>
  
 
<source lang='sql' class='def'>
 
<source lang='sql' class='def'>
−
SELECT region, name, population FROM bbc x
+
SELECT continent, name, population FROM world x
 
   WHERE population >= ALL
 
   WHERE population >= ALL
−
     (SELECT population FROM bbc y
+
     (SELECT population FROM world y
−
         WHERE y.region=x.region
+
         WHERE y.continent=x.continent
 
           AND population>0)
 
           AND population>0)
 
</source>
 
</source>
  
 
<source lang='sql' class='ans'>
 
<source lang='sql' class='ans'>
−
SELECT region, name, population FROM bbc x
+
SELECT continent, name, population FROM world x
 
   WHERE population >= ALL
 
   WHERE population >= ALL
−
     (SELECT population FROM bbc y
+
     (SELECT population FROM world y
−
         WHERE y.region=x.region
+
         WHERE y.continent=x.continent
 
           AND population>0)
 
           AND population>0)
 
</source>
 
</source>
Line 125: Line 147:
  
 
<div class='qu'>
 
<div class='qu'>
−
<p class='imper'>Find each country that belongs to a region where all populations are less than 25000000. Show '''name''', '''region''' 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>
  
 
<source lang='sql' class='def'>
 
<source lang='sql' class='def'>
Line 132: Line 154:
  
 
<source lang='sql' class='ans'>
 
<source lang='sql' class='ans'>
−
SELECT name,region,population FROM bbc x
+
SELECT name,continent,population FROM world x
 
   WHERE 25000000 >= ALL (
 
   WHERE 25000000 >= ALL (
−
     SELECT population FROM bbc y
+
     SELECT population FROM world y
−
     WHERE x.region=y.region
+
     WHERE x.continent=y.continent
 
       AND y.population>0)
 
       AND y.population>0)
 
</source>
 
</source>
Line 141: Line 163:
  
 
<div class='qu'>
 
<div class='qu'>
−
<p class='imper'>Some countries have populations more than three times that of any of their neighbours (in the same region). Give the countries and regions.</p>
+
<p class='imper'>Some countries have populations more than three times that of any of their neighbours (in the same continent). Give the countries and continents.</p>
  
 
<source lang='sql' class='def'>
 
<source lang='sql' class='def'>
Line 148: Line 170:
  
 
<source lang='sql' class='ans'>
 
<source lang='sql' class='ans'>
−
SELECT name, region FROM bbc x WHERE
+
SELECT name, continent FROM world x WHERE
 
  population > ALL
 
  population > ALL
−
  (SELECT population*3 FROM bbc y
+
  (SELECT population*3 FROM world y
−
  WHERE y.region = x.region
+
  WHERE y.continent = x.continent
 
  AND y.name != x.name)
 
  AND y.name != x.name)
 
</source>
 
</source>
 +
</div>
 +
 +
<div>
 +
<div class="lsclear">Clear your results</div>
 +
<p><div class="quizlink">[[Nested SELECT Quiz]]</div></p>
 
</div>
 
</div>

Revision as of 21:43, 8 December 2012

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='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 Algeria?

 
SELECT name FROM world WHERE
 population >
 (SELECT population
  FROM world WHERE name='Canada') 
    AND
  population <
 (SELECT population
  FROM world WHERE name='Algeria')


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 MAX(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 in each continent, show the continent, the name and the population: 

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


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

Nested SELECT Quiz

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