Difference between revisions of "SELECT within SELECT Tutorial"

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(Add explanatory hint for correlated subqueries.)
 
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==Exercises==
 
==Exercises==
 
+
==Bigger than Russia==
 
<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 that of 'Russia'. </p>
 
<pre space="preserve">world(name, continent, area, population, gdp)</pre>
 
<pre space="preserve">world(name, continent, area, population, gdp)</pre>
  
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</source>
 
</source>
 
</div>
 
</div>
 +
{{#ev:youtube|UPFwlsJFcsw}}
  
 
+
==Richer than UK==
 
<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'>Show the countries in Europe with a per capita GDP greater than 'United Kingdom'.</p>
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</source>
 
</source>
 
</div>
 
</div>
 +
{{#ev:youtube|RELAqb4DlvY}}
  
 +
==Neighbours of Argentina and Australia==
 
<div class='qu'>
 
<div class='qu'>
 
<span class='param respectorder'></span>
 
<span class='param respectorder'></span>
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   FROM world
 
   FROM world
 
   WHERE name IN ('Australia','Argentina'))
 
   WHERE name IN ('Australia','Argentina'))
 +
ORDER BY name
 
</source>
 
</source>
 
</div>
 
</div>
  
 
+
==Between Canada and Poland==
 
<div class='qu'>
 
<div class='qu'>
 
<p class='imper'>Which country has a population that is more than Canada but less than Poland? Show the name and the population.</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>
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</div>
 
</div>
  
 
+
==Percentages of Germany==
 
<div class='qu'>
 
<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.
 
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.
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</source>
 
</source>
 
</div>
 
</div>
 +
{{#ev:youtube|lCbLvHUNBG4}}
  
  
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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.
  
We can use the word <code>ALL</code> to allow >= or > or < or <=to act over a list.
+
We can use the word <code>ALL</code> to allow >= or > or < or <=to act over a list. For example, you can find the largest country in the world, by population with this query:
 +
<source lang=sql>
 +
SELECT name
 +
  FROM world
 +
WHERE population >= ALL(SELECT population
 +
                          FROM world
 +
                          WHERE population>0)
 +
</source>
 +
You need the condition '''population>0''' in the sub-query as some countries have '''null''' for population.
  
 +
==Bigger than every country in Europe==
 
<div class='qu'>
 
<div class='qu'>
 
<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>
 
<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>
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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.
 
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.
  
 +
==Largest in each continent==
 
<div class='qu'>
 
<div class='qu'>
 
<p class='imper'>Find the largest country (by area) 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>
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         and area > 0 )
 
         and area > 0 )
 
</source>
 
</source>
 +
 +
The above example is known as a '''correlated''' or '''synchronized''' sub-query.
 +
<div class='hint' title='Using correlated subqueries'>
 +
A correlated subquery works like a nested loop: the subquery only has access to rows related to a single record at a time in the outer query. The technique relies on table aliases to identify two different uses of the same table, one in the outer query and the other in the subquery.
 +
 +
One way to interpret the line in the '''WHERE''' clause that references the two table is ''“… where the correlated values are the same”''.
 +
 +
In the example provided, you would say ''“select the country details from world where the population is greater than or equal to the population of all countries where the continent is the same”''.
 +
</div>
 
</div>
 
</div>
  
 +
==First country of each continent (alphabetically)==
 +
<div class='qu'>
 +
<p class='imper'>List each continent and the name of the country that comes first alphabetically.</p>
 +
 +
<source lang='sql' class='def'>
 +
 +
</source>
 +
 +
<source lang='sql' class='ans'>
 +
SELECT continent,name FROM world x
 +
  WHERE x.name <= ALL (
 +
    SELECT name FROM world y
 +
    WHERE x.continent=y.continent)
 +
</source>
 +
</div>
  
 
==Difficult Questions That Utilize Techniques Not Covered In Prior Sections==
 
==Difficult Questions That Utilize Techniques Not Covered In Prior Sections==

Latest revision as of 14:54, 2 November 2017

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

Bigger than Russia

List each country name where the population is larger than that of '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')

Richer than UK

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')

Neighbours of Argentina and Australia

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

Between Canada and Poland

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')

Percentages of Germany

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. For example, you can find the largest country in the world, by population with this query:

SELECT name
  FROM world
 WHERE population >= ALL(SELECT population
                           FROM world
                          WHERE population>0)

You need the condition population>0 in the sub-query as some countries have null for population.

Bigger than every country in Europe

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.

Largest in each continent

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 )

The above example is known as a correlated or synchronized sub-query.

A correlated subquery works like a nested loop: the subquery only has access to rows related to a single record at a time in the outer query. The technique relies on table aliases to identify two different uses of the same table, one in the outer query and the other in the subquery.

One way to interpret the line in the WHERE clause that references the two table is “… where the correlated values are the same”.

In the example provided, you would say “select the country details from world where the population is greater than or equal to the population of all countries where the continent is the same”.

First country of each continent (alphabetically)

List each continent and the name of the country that comes first alphabetically.

SELECT continent,name FROM world x
  WHERE x.name <= ALL (
    SELECT name FROM world y
     WHERE x.continent=y.continent)

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)