databases-week1-exercises.sql

-- 1. What are the names of countries with population greater than 8 million?
SELECT name
FROM country
WHERE population > 8000000;

-- 2. What are the names of countries that have “land” in their names?
SELECT name 
FROM country
WHERE name LIKE '%land%';

-- 3. What are the names of the cities with population in between 500,000 and 1 million?
SELECT name
FROM city
WHERE population BETWEEN 500000 AND 1000000;

-- 4. What's the name of all the countries on the continent ‘Europe’?
SELECT name
FROM country 
WHERE continent = 'Europe';

-- 5. What are the names of all the cities in the Netherlands?
SELECT name
FROM city
WHERE CountryCode  = 'NLD';

-- 6. What is the population of Rotterdam?
SELECT population
FROM city 
WHERE name = 'Rotterdam';

-- 7. Which countries don't have a head of state? Hint: looks for NULL and '' values
SELECT name
FROM country
WHERE HeadOfState IS NULL OR HeadOfState =''; 

-- 8. What's the top 10 least populated cities? Return the name and population
SELECT name, population
FROM city 
ORDER BY population 
DESC LIMIT 10;

-- 9. What countries in Africa have the local name the same as their common name?
SELECT name
FROM country 
WHERE Continent = 'Africa' AND name = localname;

-- 10. What countries have Spanish as official language? Hint: see countrylanguage table
SELECT country.name
FROM country
JOIN countrylanguage ON country.code = countrylanguage.countrycode
WHERE countrylanguage.language = 'Spanish';

-- 11. What countries have official languages spoken between 1% and 10% of the population?
SELECT country.name
FROM country
JOIN countrylanguage ON country.code = countrylanguage.countrycode
WHERE countrylanguage.isofficial = 'T' AND countrylanguage.percentage BETWEEN 1 AND 10;

-- 12. What languages are spoken by over 90% of the population of a country? Return just the language names, but don't repeat entries
SELECT DISTINCT language
FROM countrylanguage
WHERE percentage > 90;

-- 13. In which countries is 'Creole English' used? Order by descending percentage of speakers
SELECT country.name, countrylanguage.percentage
FROM country
JOIN countrylanguage ON country.code = countrylanguage.countrycode
WHERE countrylanguage.language = 'Creole English'
ORDER BY countrylanguage.percentage DESC;

-- 14. What are the 5 oldest countries (by independence date) with some form of republic government? Tip: there are multiple types of republic
SELECT name, indepyear
FROM country
WHERE governmentform LIKE '%republic%'
ORDER BY indepyear
LIMIT 5;

-- 15. For each country, how many people speak each language? Important: we want absolute values, not a percentage. Return the name of the country, the name of the language, and number of speakers of that language - Hint: you need both the country and countrylanguage tables - Hint: you can do calculations between columns, for example (SELECT a - b from table;)
SELECT name, language, (Percentage / 100) * Population AS 'number' 
FROM country c 
JOIN countrylanguage c2 ON c.Code = c2.CountryCode;


-- BONUS
-- 1. What is the total population of the world?
SELECT SUM(population) AS total_population
FROM country;

-- 2. What is the average population of countries in Europe?
SELECT AVG(population) AS average_population
FROM country
WHERE continent = 'Europe';

-- 3. How many *official* languages are spoken in Belgium (`BEL`)? Return the country code, and the number of languages as "Number of Languages"
SELECT countrycode, COUNT(language) AS number_of_languages
FROM countrylanguage
WHERE countrycode = 'BEL' AND isofficial = 'T'
GROUP BY countrycode;

My answers are here: https://gist.github.com/juryp/35d88fe136a55f62590b4fc8d0185b25

-- 8. What's the top 10 least populated cities? Return the name and population
SELECT name, population
FROM city 
ORDER BY population 
DESC LIMIT 10;

you are selecting the most populated

-- 10. What countries have Spanish as official language? Hint: see countrylanguage table
SELECT country.name
FROM country
JOIN countrylanguage ON country.code = countrylanguage.countrycode
WHERE countrylanguage.language = 'Spanish';

you are selecting any country that has Spanish as language even if not official

-- 3. How many *official* languages are spoken in Belgium (`BEL`)? Return the country code, and the number of languages as "Number of Languages"
SELECT countrycode, COUNT(language) AS number_of_languages
FROM countrylanguage
WHERE countrycode = 'BEL' AND isofficial = 'T'
GROUP BY countrycode;

GROUP BY is not needed