Difference between revisions of "SELECT within SELECT Tutorial"
|Line 125:||Line 125:|
Revision as of 08:32, 7 May 2015
This tutorial looks at how we can use SELECT statements within SELECT statements to perform more complex queries.
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 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.
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.
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 )
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)