Mastering SQL with SQLZoo: A Comprehensive Guide to Solving SQL Challenges : SELECT from WORLD
Welcome to My in-depth guide on solving SQL challenges with SQLZoo! Whether you’re a budding data enthusiast, a seasoned developer, or just someone keen to sharpen your SQL skills, this blog is tailored for you. SQLZoo is a renowned interactive platform that offers a wide range of exercises designed to test and improve your understanding of SQL.
In this blog, I will walk you through various SQLZoo challenges, providing detailed solutions and explanations for each query. My aim is to help you grasp the nuances of SQL, from basic SELECT statements to more complex JOINs and subqueries. By following along, you will not only learn how to write efficient SQL queries but also gain insights into best practices and common pitfalls to avoid.
Here’s what you can expect:
- Step-by-Step Solutions: We will break down each problem, explaining the logic behind the query and the reasoning for each step.
- Comprehensive Explanations: Each solution will be accompanied by thorough explanations to ensure you understand the underlying concepts.
- Practical Tips: Along the way, we’ll share tips and tricks to help you write cleaner, more efficient SQL code.
- Varied Difficulty Levels: The challenges range from beginner to advanced, making this guide suitable for learners at any stage.
Join us on this journey to master SQL and become proficient in handling real-world data tasks with confidence. Let’s dive into the world of SQLZoo and start solving some exciting challenges!
Previous Blog Posts
1. Introduction
Read the notes about this table. Observe the result of running this SQL command to show the name, continent and population of all countries.
SELECT name, continent, population FROM world
2. Large Countries
How to use WHERE to filter records. Show the name for the countries that have a population of at least 200 million. 200 million is 200000000, there are eight zeros.
SELECT name FROM world
WHERE population > 200000000
3. Per capita GDP
Give the name and the per capita GDP for those countries with a population of at least 200 million
SELECT name, (gdp/population) as per_capita_GDP FROM world
WHERE population > 200000000
4. South America In millions
Show the name and population in millions for the countries of the continent 'South America'. Divide the population by 1000000 to get population in millions.
select name , (population/1000000) from world
where continent = 'South America'
5. France, Germany, Italy
Show the name and population for France, Germany, Italy
select name , population from world
where name in ('France', 'Germany', 'Italy')
6. United
Show the countries which have a name that includes the word 'United'
select name from world where name like '%United%'
7. Two ways to be big
Two ways to be big: A country is big if it has an area of more than 3 million sq km or it has a population of more than 250 million.
Show the countries that are big by area or big by population. Show name, population and area.
select name , population, area from world
where area > 3000000 or population > 250000000
8. One or the other (but not both)
Exclusive OR (XOR). Show the countries that are big by area (more than 3 million) or big by population (more than 250 million) but not both. Show name, population and area.
- Australia has a big area but a small population, it should be included.
- Indonesia has a big population but a small area, it should be included.
- China has a big population and big area, it should be excluded.
- United Kingdom has a small population and a small area, it should be excluded.
select name, population,area from world
WHERE (area > 3000000) <> (population > 250000000)
9. Rounding
Show the name and population in millions and the GDP in billions for the countries of the continent 'South America'. Use the ROUND function to show the values to two decimal places.
For Americas show population in millions and GDP in billions both to 2 decimal places.
select name,
ROUND((population/1000000.00),2) as population,
ROUND((gdp/1000000000.00),2) as gdp
from world
where continent = 'South America'
10. Trillion dollar economies
Show the name and per-capita GDP for those countries with a GDP of at least one trillion (1000000000000; that is 12 zeros). Round this value to the nearest 1000.
Show per-capita GDP for the trillion dollar countries to the nearest $1000.
select name ,ROUND((gdp/population),-3) from world
where gdp > 1000000000000 
11. Name and capital have the same length
Greece has capital Athens.
Each of the strings ‘Greece’, and ‘Athens’ has 6 characters.
Show the name and capital where the name and the capital have the same number of characters.
- You can use the LENGTH function to find the number of characters in a string
SELECT name, capital
FROM world
WHERE LENGTH(capital) = LENGTH(name)
12. Matching name and capital
The capital of Sweden is Stockholm. Both words start with the letter ‘S’.
Show the name and the capital where the first letters of each match. Don’t include countries where the name and the capital are the same word.
- You can use the function LEFT to isolate the first character.
- You can use
<>as the NOT EQUALS operator.
SELECT name, capital
FROM world
where (LEFT(name,1) = LEFT(capital,1)) <> (name = capital)
13. All the vowels
Equatorial Guinea and Dominican Republic have all of the vowels (a e i o u) in the name. They don’t count because they have more than one word in the name.
Find the country that has all the vowels and no spaces in its name.
- You can use the phrase
name NOT LIKE '%a%'to exclude characters from your results. - The query shown misses countries like Bahamas and Belarus because they contain at least one ‘a’
SELECT name
FROM world
WHERE name LIKE '%a%' and
name LIKE '%i%' and
name LIKE '%e%' and
name LIKE '%o%' and
name LIKE '%u%'AND
name NOT LIKE '% %'
Thank you for joining me in this SQL adventure! I hope you found the solutions and explanations in SELECT from World insightful and engaging. But we’re not stopping here. There’s so much more to explore and learn!
In SELECT from Nobel , we will delve deeper into more complex SQL challenges on SQLZoo.
Thanks :)