Difference between revisions of "SELECT within SELECT Tutorial"

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==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">
 
<div class="ref_section">
Line 36: Line 35:
 
   WHERE population >
 
   WHERE population >
 
     (SELECT population FROM world
 
     (SELECT population FROM world
       WHERE name='Russia')
+
       WHERE name='Romania')
 
</source>
 
</source>
  
Line 46: Line 45:
 
</source>
 
</source>
 
</div>
 
</div>
<div class='qu'>
 
<p class='imper'>List the '''name''' and '''continent''' of countries in the continents containing 'Belize', 'Belgium'.</p>
 
  
 +
 +
<div class='qu'>
 +
<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 55:
  
 
<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>
+
<span class='param respectorder'></span>
<div class='hint' title='Per Capita GDP'>The per capita GDP is the gdp/population</div>
+
<p class='imper'>List the '''name''' and '''continent''' of countries in the continents containing either '''Argentina''' or '''Australia'''. Order by name of the country.</p>
 +
 
 
<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 ('Australia','Argentina'))
 +
ORDER BY name
 
</source>
 
</source>
 
</div>
 
</div>
Line 90: Line 93:
 
AND
 
AND
 
(SELECT population-1 FROM world WHERE name='Poland')
 
(SELECT population-1 FROM world WHERE name='Poland')
 +
</source>
 +
</div>
 +
 +
 +
<div class='qu'>
 +
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.
 +
<p class='imper'>Show the name and the population of each country in Europe. Show the population as a percentage of the population of Germany.</p>
 +
<div class=hint title='Decimal places'>You can use the function [[ROUND]] to remove the decimal places.</div>
 +
<div class=hint title='Percent symbol %'>You can use the function [[CONCAT]] to add the percentage symbol.</div>
 +
<source lang='sql' class='def'></source>
 +
 +
<source lang='sql' class='ans'>
 +
SELECT name,
 +
  CONCAT(ROUND(100*population/(SELECT population FROM world WHERE name='Germany')),'%')
 +
FROM world
 +
WHERE continent='Europe'
 
</source>
 
</source>
 
</div>
 
</div>
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<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 130: Line 149:
  
 
<source lang='sql' class='ans'>
 
<source lang='sql' class='ans'>
SELECT continent, name, area FROM world x
+
SELECT continent, name, area  
 +
FROM world x
 
   WHERE area >= ALL
 
   WHERE area >= ALL
 
     (SELECT area FROM world y
 
     (SELECT area FROM world y
         WHERE y.continent=x.continent)
+
         WHERE y.continent=x.continent
 +
        and area > 0 )
 
</source>
 
</source>
 
</div>
 
</div>
  
  
==Difficult Questions==
+
==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 the continents where all countries have a population <= 25000000.  Then find the names of the countries associated with these continents. Show '''name''', '''continent''' and '''population'''. </p>
  
 
<source lang='sql' class='def'>
 
<source lang='sql' class='def'>
Line 172: Line 193:
  
 
<div>
 
<div>
<div class="lsclear">Clear your results</div>
 
 
<p><div class="quizlink">[[Nested SELECT Quiz]]</div></p>
 
<p><div class="quizlink">[[Nested SELECT Quiz]]</div></p>
 
</div>
 
</div>

Latest revision as of 09:49, 5 May 2015

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 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'))
ORDER BY name


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')


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.

You can use the function ROUND to remove the decimal places.
You can use the function CONCAT to add the percentage symbol.
 
SELECT name,
   CONCAT(ROUND(100*population/(SELECT population FROM world WHERE name='Germany')),'%')
FROM world
WHERE continent='Europe'


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
         AND area > 0 )


Difficult Questions That Utilize Techniques Not Covered In Prior Sections

Find the continents where all countries have a population <= 25000000. Then find the names of the countries associated with these continents. 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)