# Difference between revisions of "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.

bbc
name region area population gdp
Afghanistan South Asia 652225 26000000
Albania Europe 28728 3200000 6656000000
Algeria Middle East 2400000 32900000 75012000000
Andorra Europe 468 64000
...
Summary

## Exercises

List each country name where the population is larger than 'Russia'.

`bbc(name, region, area, population, gdp)`

```SELECT name FROM bbc
WHERE population >
(SELECT population FROM bbc
WHERE name='Russia')
```
```SELECT name FROM bbc
WHERE population >
(SELECT population FROM bbc
WHERE name='Russia')
```

List the name and region of countries in the regions containing 'India', 'Iran'.

```SELECT name,region FROM bbc
WHERE region IN
(SELECT region FROM bbc
WHERE name IN ('India','Iran'))
```

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 bbc
WHERE region='Europe' AND gdp/population >
(SELECT gdp/population FROM bbc
WHERE name='United Kingdom')
```

Which country has a population that is more than Canada but less than Algeria?

```SELECT name FROM bbc WHERE
population >
(SELECT population
FROM bbc WHERE name='Canada')
AND
population <
(SELECT population
FROM bbc WHERE name='Algeria')
```

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 bbc
WHERE gdp > ALL
(SELECT MAX(gdp) FROM bbc
WHERE region = '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 region, show the region, the name and the population:

```SELECT region, name, population FROM bbc x
WHERE population >= ALL
(SELECT population FROM bbc y
WHERE y.region=x.region
AND population>0)
```
```SELECT region, name, population FROM bbc x
WHERE population >= ALL
(SELECT population FROM bbc y
WHERE y.region=x.region
AND population>0)
```

The following questions are very difficult:

Find each country that belongs to a region where all populations are less than 25000000. Show name, region and population.

```SELECT name,region,population FROM bbc x
WHERE 25000000 >= ALL (
SELECT population FROM bbc y
WHERE x.region=y.region
AND y.population>0)
```

Some countries have populations more than three times that of any of their neighbours (in the same region). Give the countries and regions.

```SELECT name, region FROM bbc x WHERE
population > ALL
(SELECT population*3 FROM bbc y
WHERE y.region = x.region
AND y.name != x.name)
```