# SELECT within SELECT Tutorial

## SELECT within SELECT

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
...
Summary

## 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'.

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')
```

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
and area > 0 )
```

## Difficult Questions That Utilize Techniques Not Covered In Prior Sections

Find all countries that belongs to a continent if each country's population is below 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)
```