LeetCode Problem 595: Big Countries

Problem Statement:

The problem "Big Countries" is a classic SQL problem on LeetCode. It requires us to retrieve information about countries based on specific criteria for their area or population. Specifically, the task is to select the name, population, and area of countries from the World table where either the area of the country is greater than or equal to 3,000,000 square kilometers or the population is greater than or equal to 25,000,000 people.

The World table is structured as follows:

Column Name	Type	Description
name	varchar(255)	The name of the country.
population	int	The population of the country.
area	int	The area of the country in square kilometers.

Problem Breakdown:

We need to fetch the name, population, and area from the table World, with the following conditions:

The area must be greater than or equal to 3,000,000 square kilometers.
OR the population must be greater than or equal to 25,000,000 people.

Solution Explanation:

To solve this problem, we can use a simple SQL query that applies the given conditions using a WHERE clause. The conditions in the WHERE clause are connected using an OR operator, as the problem specifies that either the area or population must satisfy the condition.

Here’s the SQL query:

SELECT name, population, area 
FROM World 
WHERE area >= 3000000 OR population >= 25000000;

How the Query Works:

SELECT name, population, area: This part of the query selects the columns name, population, and area from the World table that we are interested in displaying.
FROM World: This specifies the table from which we are querying the data, which is the World table in this case.
WHERE area >= 3000000 OR population >= 25000000: This is the condition that filters the countries based on their area or population. The OR operator ensures that the condition is true if either the area is greater than or equal to 3,000,000 or the population is greater than or equal to 25,000,000.

Why This Query Works:

Area >= 3000000: Filters countries with an area of 3,000,000 square kilometers or more.
Population >= 25000000: Filters countries with a population of 25,000,000 or more.
The OR operator ensures that a country is selected if either condition is true.

This is an efficient query for retrieving the required data and works well within the constraints of the problem.

Edge Cases to Consider:

Countries with both a large area and population: These countries will be selected, as they satisfy both conditions.
Countries with only one of the conditions met: If a country has either a large area or a large population, it will still be selected.

Example:

name	population	area
China	1000000000	9596960
United States	327000000	9372610
Russia	146000000	17098242

In this example:

China and Russia are selected because they both have a large area.
The United States is selected because it has a large population.

Time Complexity:

Time complexity: O(n), where n is the number of records in the World table. The query scans through the table once to apply the conditions.
Space complexity: O(1), as the query doesn’t require any additional space beyond the result set.

Conclusion:

The solution to the "Big Countries" problem on LeetCode is a straightforward SQL query that applies conditions using a WHERE clause with the OR operator. It efficiently filters countries based on their area and population and retrieves the necessary information. Understanding how to write simple conditional queries is a key skill in SQL that is useful for solving many database-related problems.

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LeetCode Problem 595: Big Countries | Solution Explained