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]] | ||
+ | |||
+ | <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"> | + | <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> | ||
+ | |||
<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> | ||
+ | <div class='qu'> | ||
+ | <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 50: | Line 71: | ||
<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 ('Belize','Belgium')) | ||
</source> | </source> | ||
</div> | </div> | ||
Line 59: | Line 82: | ||
<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> | ||
<source lang='sql' class='ans'> | <source lang='sql' class='ans'> | ||
− | SELECT name FROM | + | 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> | </div> | ||
− | [[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 85: | Line 103: | ||
<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 110: | ||
<source lang='sql' class='ans'> | <source lang='sql' class='ans'> | ||
− | SELECT name FROM | + | SELECT name FROM world |
− | (SELECT | + | WHERE gdp > ALL |
− | WHERE | + | (SELECT gdp FROM world |
+ | WHERE continent = 'Europe' | ||
+ | AND gdp IS NOT NULL) | ||
</source> | </source> | ||
</div> | </div> | ||
Line 101: | Line 121: | ||
<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 FROM world x |
− | WHERE | + | WHERE area >= ALL |
− | (SELECT | + | (SELECT area FROM world y |
− | WHERE y. | + | WHERE y.continent=x.continent) |
− | |||
</source> | </source> | ||
</div> | </div> | ||
− | + | ==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 | + | <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 131: | Line 149: | ||
<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 140: | Line 158: | ||
<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 147: | Line 165: | ||
<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> | ||
+ | <div class="lsclear">Clear your results</div> | ||
+ | <p><div class="quizlink">[[Nested SELECT Quiz]]</div></p> | ||
</div> | </div> |
Revision as of 18:19, 19 May 2014
SELECT within SELECT
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 |
... |
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'.
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 'Belize', 'Belgium'.
SELECT name,continent
FROM world
WHERE continent IN (
SELECT continent
FROM world
WHERE name IN ('Belize','Belgium'))
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')
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)
Difficult Questions That Utilize Techniques Not Covered In Prior Sections
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)