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"> | ||
+ | <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]] | ||
+ | |||
+ | <div class="progress_panel"><div> | ||
+ | <div class="summary">Summary</div> | ||
+ | <div class="progressbarbg"> | ||
+ | <div class="progressbar"></div> | ||
+ | </div> | ||
+ | </div></div> | ||
− | |||
==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"> | + | <pre space="preserve">world(name, continent, area, population, gdp)</pre> |
<source lang='sql' class='def'> | <source lang='sql' class='def'> | ||
− | SELECT name FROM | + | SELECT name FROM world |
WHERE population > | WHERE population > | ||
− | (SELECT population FROM | + | (SELECT population FROM world |
− | WHERE name=' | + | WHERE name='Romania') |
</source> | </source> | ||
<source lang='sql' class='ans'> | <source lang='sql' class='ans'> | ||
− | SELECT name FROM | + | SELECT name FROM world |
WHERE population > | WHERE population > | ||
− | (SELECT population FROM | + | (SELECT population FROM world |
WHERE name='Russia') | WHERE name='Russia') | ||
</source> | </source> | ||
</div> | </div> | ||
+ | {{#ev:youtube|UPFwlsJFcsw}} | ||
+ | ==Richer than UK== | ||
<div class='qu'> | <div class='qu'> | ||
− | <p class='imper'> | + | <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 33: | Line 56: | ||
<source lang='sql' class='ans'> | <source lang='sql' class='ans'> | ||
− | SELECT name | + | SELECT name FROM world |
− | WHERE | + | WHERE continent='Europe' AND gdp/population > |
− | + | (SELECT gdp/population FROM world | |
− | + | WHERE name='United Kingdom') | |
</source> | </source> | ||
</div> | </div> | ||
+ | {{#ev:youtube|RELAqb4DlvY}} | ||
+ | ==Neighbours of Argentina and Australia== | ||
+ | <div class='qu'> | ||
+ | <span class='param respectorder'></span> | ||
+ | <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 50: | Line 74: | ||
<source lang='sql' class='ans'> | <source lang='sql' class='ans'> | ||
− | SELECT name FROM | + | SELECT name,continent |
− | + | FROM world | |
− | + | WHERE continent IN ( | |
− | + | SELECT continent | |
+ | FROM world | ||
+ | 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 | + | <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 lang='sql' class='def'></source> |
+ | <source lang='sql' class='ans'> | ||
+ | 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') | ||
</source> | </source> | ||
+ | </div> | ||
+ | |||
+ | ==Percentages of Germany== | ||
+ | <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'> | <source lang='sql' class='ans'> | ||
− | SELECT name | + | SELECT name, |
− | + | CONCAT(ROUND(100*population/(SELECT population FROM world WHERE name='Germany')),'%') | |
− | + | FROM world | |
− | + | WHERE continent='Europe' | |
− | |||
− | |||
− | |||
− | |||
</source> | </source> | ||
</div> | </div> | ||
+ | {{#ev:youtube|lCbLvHUNBG4}} | ||
− | [[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. | ||
− | 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 | + | <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 92: | Line 140: | ||
<source lang='sql' class='ans'> | <source lang='sql' class='ans'> | ||
− | SELECT name FROM | + | SELECT name FROM world |
WHERE gdp > ALL | WHERE gdp > ALL | ||
− | (SELECT | + | (SELECT gdp FROM world |
− | WHERE | + | WHERE continent = 'Europe' |
AND gdp IS NOT NULL) | AND gdp IS NOT NULL) | ||
</source> | </source> | ||
Line 102: | Line 150: | ||
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 in each | + | <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'> | ||
− | SELECT | + | SELECT continent, name, population FROM world x |
WHERE population >= ALL | WHERE population >= ALL | ||
− | (SELECT population FROM | + | (SELECT population FROM world y |
− | WHERE y. | + | WHERE y.continent=x.continent |
AND population>0) | AND population>0) | ||
</source> | </source> | ||
<source lang='sql' class='ans'> | <source lang='sql' class='ans'> | ||
− | SELECT | + | SELECT continent, name, area |
− | WHERE | + | FROM world x |
− | (SELECT | + | WHERE area >= ALL |
− | WHERE y. | + | (SELECT area FROM world y |
− | + | WHERE y.continent=x.continent | |
+ | 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== | ||
<div class='qu'> | <div class='qu'> | ||
− | <p class='imper'>Find | + | <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 133: | Line 206: | ||
<source lang='sql' class='ans'> | <source lang='sql' class='ans'> | ||
− | SELECT name, | + | SELECT name,continent,population FROM world x |
WHERE 25000000 >= ALL ( | WHERE 25000000 >= ALL ( | ||
− | SELECT population FROM | + | SELECT population FROM world y |
− | WHERE x. | + | WHERE x.continent=y.continent |
AND y.population>0) | AND y.population>0) | ||
</source> | </source> | ||
Line 142: | Line 215: | ||
<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 | + | <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 149: | Line 222: | ||
<source lang='sql' class='ans'> | <source lang='sql' class='ans'> | ||
− | SELECT name, | + | SELECT name, continent FROM world x WHERE |
population > ALL | population > ALL | ||
− | (SELECT population*3 FROM | + | (SELECT population*3 FROM world y |
− | WHERE y. | + | WHERE y.continent = x.continent |
AND y.name != x.name) | AND y.name != x.name) | ||
</source> | </source> | ||
+ | </div> | ||
+ | |||
+ | <div> | ||
+ | <p><div class="quizlink">[[Nested SELECT Quiz]]</div></p> | ||
</div> | </div> |
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.
name | continent | area | population | gdp |
---|---|---|---|---|
Afghanistan | Asia | 652230 | 25500100 | 20343000000 |
Albania | Europe | 28748 | 2831741 | 12960000000 |
Algeria | Africa | 2381741 | 37100000 | 188681000000 |
Andorra | Europe | 468 | 78115 | 3712000000 |
Angola | Africa | 1246700 | 20609294 | 100990000000 |
... |
Contents
- 1 Exercises
- 2 Bigger than Russia
- 3 Richer than UK
- 4 Neighbours of Argentina and Australia
- 5 Between Canada and Poland
- 6 Percentages of Germany
- 7 Bigger than every country in Europe
- 8 Largest in each continent
- 9 First country of each continent (alphabetically)
- 10 Difficult Questions That Utilize Techniques Not Covered In Prior Sections
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'.
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.
SELECT name,
CONCAT(ROUND(100*population/(SELECT population FROM world WHERE name='Germany')),'%')
FROM world
WHERE continent='Europe'
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)