databases-week1-exercises.sql

-- 1. What are the names of countries with population greater than 8 million?
SELECT Name , population 
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, population
from city 
where Population > 500000 and Population <1000000 ; 

-- 4. What's the name of all the countries on the continent ‘Europe’?
SELECT name, continent 
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 name, population
from city 
where Name = 'Rotterdam';

-- 7. Which countries don't have a head of state? Hint: looks for NULL and '' values
select name, headofstate
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 
limit 10 ; 

-- 9. What countries in Africa have the local name the same as their common name?
SELECT name,LocalName, continent 
FROM country
WHERE Continent = 'africa' and Name = LocalName ;

-- 10. What countries have Spanish as official language? Hint: see countrylanguage table
select country.Name , countrylanguage.language , countrylanguage.IsOfficial
from country join countrylanguage on country.code = countrylanguage.CountryCode 
where language = 'spanish' and isOfficial = 't';

-- 11. What countries have official languages spoken between 1% and 10% of the population?
select country.Name , countrylanguage.language , countrylanguage.IsOfficial, countrylanguage.Percentage
from country join countrylanguage on country.code = countrylanguage.CountryCode 
where countrylanguage.Percentage > 1 and countrylanguage.Percentage < 10 and isOfficial = 't';

    or ---->
select Name , language , IsOfficial, Percentage
from country join countrylanguage on code = CountryCode 
where isOfficial = 't' and 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 Name , language , Percentage
from country join countrylanguage on code = CountryCode
where language = 'Creole English'
order by 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 
from country 
where IndepYear IS NOT null
and 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, floor(Population *percentage / 100 ) as 'language speakers'
from country join countrylanguage on Code = countrycode;



-- BONUS
-- 1. What is the total population of the world?
select sum(population) from country ; 

-- 2. What is the average population of countries in Europe?
SELECT avg(population) from country ;

-- 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';



-- MORE
-- 1. Retrieve the names of all the cities along with their respective country names, make sure the column names are easy to understand.
select city.Name as 'city' , country.name as 'country' 
from city join country 
where city.CountryCode =country.Code ;

-- 2. Find the average life expectancy of countries in the continent 'Europe'.
select avg(lifeExpectancy) 
from country
where Continent = 'europe';

-- 3. Get the names and populations of cities in the district 'California'.
select name , population 
from city 
where District = 'California';

-- 4. Retrieve the capital city of each country along with the country name.
SELECT  city.Name as 'capital' , country.Name as 'country'
from city join country on Country.Capital = city.ID ;

-- 5. Find the largest city by population in each country.
SELECT  Country.Name,  MAX(city.Population) as MaxPopulation 
FROM city join country  on city.CountryCode = Country.Code 
GROUP BY CountryCode;

-- 6. List the names of all cities with a population of over 1 million in the continent of 'Asia'.
SELECT  city.name , city.population, Country.Continent
from city join country on city.CountryCode = Country.Code
where city.Population > 1000000 and Country.Continent = 'asia';


-- 7. Get the names and continents of countries that do not have an official language recorded in the database.
SELECT country.name AS Country, country.Continent
FROM country
WHERE country.Code NOT IN (
    SELECT CountryCode
    FROM countrylanguage
    WHERE IsOfficial = 'T'
);
-- or ---->
SELECT country.Name, country.Continent
FROM country 
left JOIN countryLanguage  ON country.Code = countryLanguage.CountryCode AND countryLanguage.IsOfficial = 'T'
WHERE countryLanguage.CountryCode IS NULL;

-- 8. List the countries in the 'Oceania' continent with an average life expectancy over 70.
SELECT  name, continent, LifeExpectancy 
from country 
where continent = 'Oceania' and LifeExpectancy > 70; 

-- 9. Find the total number of languages spoken in the continent 'Africa'.
select  count(distinct countrylanguage.language) as 'total Languages in africa' , Country.Continent 
from countrylanguage join  Country 
on countrylanguage.CountryCode= country.Code
where Country.Continent = 'africa';

-- 10. Retrieve the names and populations of cities located in countries with a surface area greater than 1 million square kilometers.
SELECT city.Name as city, city.Population, Country.Name as country 
from city join country on city.CountryCode  = Country.Code
where Country.SurfaceArea > 1000000 ; 


-- 11. Retrieve the names and populations of capital cities with populations over 500,000.
SELECT city.Name as city, city.Population
from city join   country on Country.Capital = city.ID
where city.Population > 500000;

-- 12. List the names and continents of countries where English is an official language.
SELECT country.name as country , country.continent
from country join countrylanguage
on country.Code = countrylanguage.CountryCode
where countrylanguage.language = 'english' and countrylanguage.IsOfficial = 't';  

-- 13. Find the name of the country with the highest life expectancy.
SELECT Name
FROM country
ORDER BY LifeExpectancy DESC
LIMIT 1;
 -- or ----> 
SELECT Name, LifeExpectancy 
FROM country
WHERE LifeExpectancy = (SELECT MAX(LifeExpectancy) FROM country);

-- 14. Get the names of countries in the 'South America' continent with cities having a population over 1 million.
select  distinct Country.Name as country
from country join city on Country.Code = city.CountryCode
where Country.Continent= 'South America' and city.Population > 1000000 ;


-- 15. Find the name and population of the smallest city (by population) in the country 'India'.
SELECT city.Name as city, city.Population , country.Name as country
from city join country 
on city.countrycode = country.Code
where country.Name  = 'india' order by population 
limit 1; 



-- 16. Retrieve the country name and its corresponding capital city's name where the capital's population is more than 1 million.
SELECT country.name as country, city.Name as capital , city.population 
from country join city 
on Country.Code = city.CountryCode 
where country.Capital = city.ID 
and city.Population >1000000 ;

-- 17. List the names of countries that have no cities in the database.
SELECT country.Name as country 
from country 
where Country.Code not in (
	select city.CountryCode
	from city 
	where country.Code = city.CountryCode 
) ;


-- 18. Get the name and population of the largest city in the continent 'South America'.
select city.Name as city, city.Population, country.Continent 
from city 
join country on city.CountryCode = Country.Code
where Country.Continent = 'South America'
order by city.Population desc 
limit 1;

select city.Name as city, city.Population, country.Continent 
from city 
join country on city.CountryCode = Country.Code
where city.Population in(
	select max(city.Population)
	from city 
	join country on city.CountryCode = Country.Code
	where Country.Continent = 'South America'
);


-- 19. List the names and populations of all cities in countries where the official language is 'Spanish'.
SELECT city.Name  as city, city.Population, countrylanguage.Language , countrylanguage.IsOfficial 
from city 
join countrylanguage 
on city.CountryCode = countrylanguage.CountryCode 
where countrylanguage.Language = 'spanish'
and countrylanguage.IsOfficial = 't';

select name as city , population 
from city 
where CountryCode in(
	select CountryCode 
	from countrylanguage 
	where language = 'spanish'
	and IsOfficial = 't'
);

-- 20. Get the name of the country and the population of the city with the highest population in that country.

SELECT country.Name as country, city.Name as city, city.Population 
from country 
join city 
on Country.Code = city.CountryCode 
where city.Population in (
	select max(Population) 
	from city
	where city.CountryCode = Country.Code 
);