SQL ZOO 练习 —— SUM and COUNT

SUM and COUNT

1.Show the total population of the world.

SELECT SUM(population)
FROM world

2.List all the continents - just once each.

SELECT DISTINCT continent 
FROM world

3.Give the total GDP of Africa

SELECT SUM(gdp) 
FROM world 
WHERE continent = 'Africa'

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

SELECT COUNT(name) 
FROM world 
WHERE area >= 1000000

5.What is the total population of (‘Estonia’, ‘Latvia’, ‘Lithuania’)

SELECT SUM(population) 
FROM world 
WHERE name IN('Estonia','Latvia','Lithuania')

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

第一种

SELECT distinct continent,(SELECT COUNT(*)
FROM world a
WHERE a.continent = b.continent ) AS number
FROM world b

第二种

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.

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.

SELECT continent
FROM world 
GROUP BY continent
HAVING SUM(population) >= 100000000

SUM and COUNT Quiz

1. Select the statement that shows the sum of population of all countries in ‘Europe’

SELECT SUM(population) FROM bbc WHERE region = 'Europe'

2. Select the statement that shows the number of countries with population smaller than 150000

SELECT COUNT(name) FROM bbc WHERE population < 150000

3. Select the list of core SQL aggregate functions

AVG(), COUNT(), MAX(), MIN(), SUM()

4. Select the result that would be obtained from the following code:

SELECT region, SUM(area)
   FROM bbc 
  WHERE SUM(area) > 15000000 
  GROUP BY region

Result：

No result due to invalid use of the WHERE function

5. Select the statement that shows the average population of ‘Poland’, ‘Germany’ and ‘Denmark’

 SELECT AVG(population) FROM bbc WHERE name IN ('Poland', 'Germany', 'Denmark')

6. Select the statement that shows the medium population density of each region

 SELECT region, SUM(population)/SUM(area) AS density FROM bbc GROUP BY region

7. Select the statement that shows the name and population density of the country with the largest population

SELECT name, population/area AS density FROM bbc WHERE population = (SELECT MAX(population) FROM bbc)

8. Pick the result that would be obtained from the following code:

SELECT region, SUM(area) 
   FROM bbc 
  GROUP BY region 
  HAVING SUM(area)<= 20000000

Result：