This document contains my solutions to the SQLZoo 'SUM and COUNT' section and the 'Nobel Prizes: Aggregate functions' tutorial using MySQL syntax, along with my personal learning notes and explanations.
Show the total population of the world.
world(name, continent, area, population, gdp)
My Solution:
SELECT SUM(population)
FROM world;My Notes:
SUM() is an aggregate function that calculates the total population across all rows.
List all the continents - just once each.
My Solution:
SELECT DISTINCT continent
FROM world;My Notes:
DISTINCT removes duplicate values, ensuring each continent appears only once.
Give the total GDP of Africa.
My Solution:
SELECT SUM(gdp)
FROM world
WHERE continent = 'Africa';My Notes:
Using SUM() with WHERE filters the sum to only African countries.
How many countries have an area of at least 1000000.
My Solution:
SELECT COUNT(name)
FROM world
WHERE area >= 1000000;My Notes:
COUNT() counts the number of rows matching the condition.
What is the total population of ('Estonia', 'Latvia', 'Lithuania').
My Solution:
SELECT SUM(population)
FROM world
WHERE name IN ('Estonia', 'Latvia', 'Lithuania');For each continent show the continent and number of countries.
My Solution:
SELECT continent, COUNT(name)
FROM world
GROUP BY continent;My Notes:
GROUP BY groups rows by continent, and COUNT() counts how many countries exist per continent.
For each continent show the continent and number of countries with populations of at least 10 million.
My Solution:
SELECT continent, COUNT(name)
FROM world
WHERE population >= 10000000
GROUP BY continent;My Notes:
Applying WHERE before GROUP BY ensures only countries meeting the population condition are counted.
List the continents that have a total population of at least 100 million.
My Solution:
SELECT continent
FROM world
GROUP BY continent
HAVING SUM(population) >= 100000000;My Notes:
The HAVING clause filters groups after GROUP BY aggregation.
Here are my solutions and notes for a related SQLZoo section, the 'Nobel Prizes: Aggregate functions' tutorial.
Show the total number of prizes awarded.
My Solution:
SELECT COUNT(winner)
FROM nobel;List each subject - just once.
My Solution:
SELECT DISTINCT subject
FROM nobel;Show the total number of prizes awarded for Physics.
nobel(yr, subject, winner)
My Solution:
SELECT COUNT(winner)
FROM nobel
WHERE subject = 'Physics';For each subject show the subject and the number of prizes.
nobel(yr,subject, winner)
My Solution:
SELECT subject, COUNT(winner)
FROM nobel
GROUP BY subject;For each subject show the first year that the prize was awarded.
nobel(yr, subject, winner)
My Solution:
SELECT subject, yr
FROM nobel x
WHERE yr <=
ALL(
SELECT yr
FROM nobel y
WHERE y.subject = x.subject
)
GROUP BY subject;For each subject show the number of prizes awarded in the year 2000.
nobel(yr, subject, winner)
My Solution:
SELECT subject, COUNT(winner)
FROM nobel
WHERE yr = 2000
GROUP BY subject;Show the number of different winners for each subject. Be aware that Frederick Sanger has won the chemistry prize twice - he should only be counted once.
nobel(yr, subject, winner)
My Solution:
SELECT subject, COUNT(DISTINCT winner)
FROM nobel
GROUP BY subject;For each subject show how many years have had prizes awarded.
nobel(yr, subject, winner)
My Solution:
SELECT subject, COUNT(DISTINCT yr)
FROM nobel
GROUP BY subject;Show the years in which three prizes were given for Physics.
nobel(yr, subject, winner)
My Solution:
SELECT yr
FROM nobel
WHERE subject = 'Physics'
GROUP BY yr
HAVING COUNT(winner) = 3;Show winners who have won more than once.
nobel(yr, subject, winner)
My Solution:
SELECT winner
FROM nobel
GROUP BY winner
HAVING COUNT(winner) > 1;Show winners who have won more than one subject.
nobel(yr, subject, winner)
My Solution:
SELECT winner
FROM nobel
GROUP BY winner
HAVING COUNT(DISTINCT subject) > 1;Show the year and subject where 3 prizes were given. Show only years 2000 onwards.
nobel(yr, subject, winner)
My Solution:
SELECT yr, subject
FROM nobel
WHERE yr >= 2000
GROUP BY yr, subject
HAVING COUNT(winner) = 3;