Difference between revisions of "SELECT within SELECT Tutorial"

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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'>
 
<div class='ref_section'>
−
<div class="ref_section">
 
 
<table class='db_ref'>
 
<table class='db_ref'>
−
<tr><th>name</th><th>continent</th><th>area</th>
+
<caption>bbc</caption>
−
<th>population</th><th>gdp</th></tr>
+
<tr>
−
<tr><td>Afghanistan</td><td>Asia</td><td class="r">652230</td><td class="r">25500100</td><td class="r">20343000000</td></tr>
+
<th>name</th>
−
<tr><td>Albania</td><td>Europe</td><td class="r">28748 </td><td class="r">2831741 </td><td class="r">12960000000 </td></tr>
+
<th>region</th>
−
<tr><td>Algeria</td><td>Africa</td><td class="r">2381741 </td><td class="r">37100000 </td><td class="r">188681000000 </td></tr>
+
<th>area</th>
−
<tr><td>Andorra</td><td>Europe</td><td class="r">468</td><td class="r">78115 </td><td class="r">3712000000 </td></tr><tr>
+
<th>population</th>
−
<td>Angola</td><td>Africa</td><td class="r">1246700 </td><td class="r">20609294 </td><td class="r">100990000000 </td></tr>
+
<th>gdp</th>
 +
</tr>
 +
<tr>
 +
<td>Afghanistan</td>
 +
<td>South Asia</td>
 +
<td align='right'>652225</td>
 +
<td align='right'>26000000</td>
 +
<td></td>
 +
</tr>
 +
<tr>
 +
<td>Albania</td>
 +
<td>Europe</td>
 +
<td align='right'>28728</td>
 +
<td align='right'>3200000</td>
 +
<td align='right'>6656000000</td>
 +
</tr>
 +
<tr>
 +
<td>Algeria</td>
 +
<td>Middle East</td>
 +
<td align='right'>2400000</td>
 +
<td align='right'>32900000</td>
 +
<td align='right'>75012000000</td>
 +
</tr>
 +
<tr>
 +
<td>Andorra</td>
 +
<td>Europe</td>
 +
<td align='right'>468</td>
 +
<td align='right'>64000</td>
 +
<td></td>
 +
</tr>
 
<tr>
 
<tr>
 
<td colspan='5'>...</td>
 
<td colspan='5'>...</td>
Line 30: Line 58:
 
<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">world(name, continent, area, population, gdp)</pre>
+
<pre space="preserve">bbc(name, region, area, population, gdp)</pre>
  
  
 
<source lang='sql' class='def'>
 
<source lang='sql' class='def'>
−
SELECT name FROM world
+
SELECT name FROM bbc
 
   WHERE population >
 
   WHERE population >
−
     (SELECT population FROM world
+
     (SELECT population FROM bbc
 
       WHERE name='Russia')
 
       WHERE name='Russia')
 
</source>
 
</source>
  
 
<source lang='sql' class='ans'>
 
<source lang='sql' class='ans'>
−
SELECT name FROM world
+
SELECT name FROM bbc
 
   WHERE population >
 
   WHERE population >
−
     (SELECT population FROM world
+
     (SELECT population FROM bbc
 
       WHERE name='Russia')
 
       WHERE name='Russia')
 
</source>
 
</source>
 
</div>
 
</div>
 
<div class='qu'>
 
<div class='qu'>
−
<p class='imper'>List the '''name''' and '''continent''' of countries in the continents containing 'India', 'Iran'.</p>
+
<p class='imper'>List the '''name''' and '''region''' of countries in the regions containing 'India', 'Iran'.</p>
  
 
<source lang='sql' class='def'>
 
<source lang='sql' class='def'>
Line 55: Line 83:
  
 
<source lang='sql' class='ans'>
 
<source lang='sql' class='ans'>
−
SELECT name,continent FROM world
+
SELECT name,region FROM bbc
−
   WHERE continent IN
+
   WHERE region IN
−
     (SELECT continent FROM world
+
     (SELECT region FROM bbc
 
     WHERE name IN ('India','Iran'))
 
     WHERE name IN ('India','Iran'))
 
</source>
 
</source>
Line 70: Line 98:
  
 
<source lang='sql' class='ans'>
 
<source lang='sql' class='ans'>
−
SELECT name FROM world
+
SELECT name FROM bbc
−
   WHERE continent='Europe' AND gdp/population >
+
   WHERE region='Europe' AND gdp/population >
−
     (SELECT gdp/population FROM world
+
     (SELECT gdp/population FROM bbc
 
       WHERE name='United Kingdom')
 
       WHERE name='United Kingdom')
 
</source>
 
</source>
Line 86: Line 114:
  
 
<source lang='sql' class='ans'>
 
<source lang='sql' class='ans'>
−
SELECT name FROM world WHERE
+
SELECT name FROM bbc WHERE
 
  population >
 
  population >
 
  (SELECT population
 
  (SELECT population
−
   FROM world WHERE name='Canada')  
+
   FROM bbc WHERE name='Canada')  
 
     AND
 
     AND
 
   population <
 
   population <
 
  (SELECT population
 
  (SELECT population
−
   FROM world WHERE name='Algeria')
+
   FROM bbc WHERE name='Algeria')
 
</source>
 
</source>
 
</div>
 
</div>
Line 112: Line 140:
  
 
<source lang='sql' class='ans'>
 
<source lang='sql' class='ans'>
−
SELECT name FROM world
+
SELECT name FROM bbc
 
   WHERE gdp > ALL
 
   WHERE gdp > ALL
−
   (SELECT MAX(gdp) FROM world
+
   (SELECT MAX(gdp) FROM bbc
−
     WHERE continent = 'Europe'  
+
     WHERE region = 'Europe'  
 
       AND gdp IS NOT NULL)
 
       AND gdp IS NOT NULL)
 
</source>
 
</source>
Line 123: Line 151:
  
 
<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 in each region, show the '''region''', the '''name''' and the '''population''': </p>
  
 
<source lang='sql' class='def'>
 
<source lang='sql' class='def'>
−
SELECT continent, name, population FROM world x
+
SELECT region, name, population FROM bbc x
 
   WHERE population >= ALL
 
   WHERE population >= ALL
−
     (SELECT population FROM world y
+
     (SELECT population FROM bbc y
−
         WHERE y.continent=x.continent
+
         WHERE y.region=x.region
 
           AND population>0)
 
           AND population>0)
 
</source>
 
</source>
  
 
<source lang='sql' class='ans'>
 
<source lang='sql' class='ans'>
−
SELECT continent, name, population FROM world x
+
SELECT region, name, population FROM bbc x
 
   WHERE population >= ALL
 
   WHERE population >= ALL
−
     (SELECT population FROM world y
+
     (SELECT population FROM bbc y
−
         WHERE y.continent=x.continent
+
         WHERE y.region=x.region
 
           AND population>0)
 
           AND population>0)
 
</source>
 
</source>
Line 146: Line 174:
  
 
<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 region where all populations are less than 25000000. Show '''name''', '''region''' and '''population'''. </p>
  
 
<source lang='sql' class='def'>
 
<source lang='sql' class='def'>
Line 153: Line 181:
  
 
<source lang='sql' class='ans'>
 
<source lang='sql' class='ans'>
−
SELECT name,continent,population FROM world x
+
SELECT name,region,population FROM bbc x
 
   WHERE 25000000 >= ALL (
 
   WHERE 25000000 >= ALL (
−
     SELECT population FROM world y
+
     SELECT population FROM bbc y
−
     WHERE x.continent=y.continent
+
     WHERE x.region=y.region
 
       AND y.population>0)
 
       AND y.population>0)
 
</source>
 
</source>
Line 162: Line 190:
  
 
<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 continent). Give the countries and continents.</p>
+
<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>
  
 
<source lang='sql' class='def'>
 
<source lang='sql' class='def'>
Line 169: Line 197:
  
 
<source lang='sql' class='ans'>
 
<source lang='sql' class='ans'>
−
SELECT name, continent FROM world x WHERE
+
SELECT name, region FROM bbc x WHERE
 
  population > ALL
 
  population > ALL
−
  (SELECT population*3 FROM world y
+
  (SELECT population*3 FROM bbc y
−
  WHERE y.continent = x.continent
+
  WHERE y.region = x.region
 
  AND y.name != x.name)
 
  AND y.name != x.name)
 
</source>
 
</source>

Revision as of 21:37, 8 December 2012

SELECT within SELECT

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

bbc
name region area population gdp
Afghanistan South Asia 652225 26000000
Albania Europe 28728 3200000 6656000000
Algeria Middle East 2400000 32900000 75012000000
Andorra Europe 468 64000
...

Using nested SELECT

Summary

Exercises

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

bbc(name, region, area, population, gdp)


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

List the name and region of countries in the regions containing 'India', 'Iran'.

 
SELECT name,region FROM bbc
  WHERE region IN
    (SELECT region FROM bbc
     WHERE name IN ('India','Iran'))

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 bbc
  WHERE region='Europe' AND gdp/population >
     (SELECT gdp/population FROM bbc
      WHERE name='United Kingdom')


Which country has a population that is more than Canada but less than Algeria?

 
SELECT name FROM bbc WHERE
 population >
 (SELECT population
  FROM bbc WHERE name='Canada') 
    AND
  population <
 (SELECT population
  FROM bbc 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 bbc 
  WHERE gdp > ALL
   (SELECT MAX(gdp) FROM bbc
    WHERE region = '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 region, show the region, the name and the population: 

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


The following questions are very difficult:

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

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

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

 
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)
Clear your results

Nested SELECT Quiz

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