595_big_countries.md

Big Countries

Problem Number: 595 Difficulty: Easy Category: Database, SQL LeetCode Link: https://leetcode.com/problems/big-countries/

Problem Description

A country is big if:

• it has an area of at least three million (i.e., 3000000 km²), or
• it has a population of at least twenty-five million (i.e., 25000000).

Write a solution to find the name, population, and area of the big countries.

Return the result table in any order.

Example:

Input: 
World table:
+-------------+-----------+---------+------------+--------------+
| name        | continent | area    | population | gdp          |
+-------------+-----------+---------+------------+--------------+
| Afghanistan | Asia      | 652230  | 25500100   | 20343000000  |
| Albania     | Europe    | 28748   | 2831741    | 12960000000  |
| Algeria     | Africa    | 2381741 | 37100000   | 188681000000 |
| Andorra     | Europe    | 468     | 78115      | 3712000000   |
| Angola      | Africa    | 1246700 | 20609294   | 100990000000 |
+-------------+-----------+---------+------------+--------------+

Output: 
+-------------+------------+---------+
| name        | population | area    |
+-------------+------------+---------+
| Afghanistan | 25500100   | 652230  |
| Algeria     | 37100000   | 2381741 |
+-------------+------------+---------+

My Approach

I used a Simple SQL Query approach with WHERE clause to filter big countries based on the given criteria. The key insight is to use OR condition to check both area and population thresholds.

Algorithm:

1. Select name, population, and area columns
2. Use WHERE clause with OR condition
3. Check if area >= 3000000 OR population >= 25000000
4. Return results in any order

Solution

The solution uses a simple SQL query with WHERE clause filtering. See the implementation in the solution file.

Key Points:

• Uses SELECT with specific columns
• Applies WHERE clause with OR condition
• Filters based on area and population thresholds
• Returns results in any order

Time & Space Complexity

Time Complexity: O(n)

• Table scan: O(n) where n is number of rows
• WHERE clause evaluation: O(n)
• Total: O(n)

Space Complexity: O(k)

• Result set: O(k) where k is number of matching rows
• In worst case: O(n)

Key Insights

1. Simple Filtering: Use WHERE clause with OR condition for multiple criteria.

2. Column Selection: Only select required columns (name, population, area).

3. Threshold Comparison: Compare area and population against specific thresholds.

4. OR Logic: Use OR to include countries that meet either condition.

5. Order Independence: Results can be returned in any order.

6. Efficient Query: Simple SELECT with WHERE is optimal for this problem.

Mistakes Made

1. Wrong Operator: Initially might use AND instead of OR.

2. Extra Columns: Might select unnecessary columns like continent or gdp.

3. Complex Logic: Overcomplicating the query with unnecessary joins or subqueries.

4. Wrong Thresholds: Not using the correct area and population thresholds.

Related Problems

• Classes More Than 5 Students (Problem 596): SQL filtering with GROUP BY
• Not Boring Movies (Problem 620): SQL filtering with conditions
• Duplicate Emails (Problem 182): SQL with GROUP BY and HAVING
• Employees Earning More Than Their Managers (Problem 181): SQL self-join

Alternative Approaches

1. UNION: Use UNION of two separate queries - O(n) time
2. CASE Statement: Use CASE in WHERE clause - O(n) time
3. Subquery: Use subquery approach - O(n) time

Common Pitfalls

1. Wrong Operator: Using AND instead of OR for the conditions.
2. Extra Columns: Selecting unnecessary columns in the result.
3. Complex Logic: Overcomplicating with unnecessary joins or subqueries.
4. Wrong Thresholds: Not using the correct area and population thresholds.
5. Performance: Not considering index usage for large tables.

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Note: This is a SQL problem that demonstrates basic filtering with WHERE clause and OR conditions.