sqlzoo练习答案--SUM and COUNT

World Country Profile: Aggregate functions

This tutorial is about aggregate functions such as COUNT, SUM and AVG. An aggregate function takes many values and delivers just one value. For example the function SUM would aggregate the values 2, 4 and 5 to deliver the single value 11.

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
...

1、 Show the total  population  of the world.

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1. SELECT SUM(population)  
2. FROM world  

2、 List all the continents - just once each.

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1. select DISTINCT(continent) from world  

3、 Give the total GDP of Africa

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1. SELECT SUM(gdp)  
2. FROM world where continent='Africa'  

4、 How many countries have an  area  of at least 1000000

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1. select count(name) from world where area>1000000  

5、 What is the total  population  of ('France','Germany','Spain')

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1. SELECT SUM(population)  
2. FROM world where name in('France','Germany','Spain')  

6、 For each  continent  show the  continent  and number of countries.

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1. select continent,count(name) from world group by continent  

7、 For each  continent  show the  continent  and number of countries with populations of at least 10 million.

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1. select continent,count(name) from world where population>= 10000000 group by continent  

8、 List the continents that  have  a total population of at least 100 million.

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1. select continent from world group by continent having sum(population)>=100000000