Difference between revisions of "SELECT within SELECT Tutorial"
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| − | <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?</p> |
| − | <source lang='sql' class='def'> | + | <source lang='sql' class='def'></source> |
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| − | </source> | + | |
<source lang='sql' class='ans'> | <source lang='sql' class='ans'> | ||
| − | SELECT name FROM world WHERE | + | 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') | |
| − | + | ||
| − | + | ||
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</source> | </source> | ||
</div> | </div> | ||
Revision as of 21:50, 8 December 2012
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='Russia')
SELECT name FROM world WHERE population > (SELECT population FROM world WHERE name='Russia')
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'))
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')
Which country has a population that is more than Canada but less than Poland?
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 any country in Europe? [Give the name only.]
SELECT name FROM world WHERE gdp > ALL (SELECT MAX(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 in each continent, show the continent, the name and the population:
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, population FROM world x WHERE population >= ALL (SELECT population FROM world y WHERE y.continent=x.continent AND population>0)
The following questions are very difficult:
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)
