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feat: Organize repository for self-guided LeetCode tutorials
This commit introduces a new structure and documentation to transform the repository into a self-guided LeetCode SQL tutorial platform. Key changes include: - **Enhanced `README.md`:** Updated the main README with a description of the tutorial format and a "How to Use This Repository" section. - **`DOCUMENTATION_GUIDE.md`:** Added a new guide that defines a standard structure for individual LeetCode problem files. This includes sections for Problem Title, LeetCode Link, Problem Description, SQL Solution, Explanation, and Key Concepts. - **Sample File Restructuring:** Reformatted three existing SQL files (`Easy/Ads performance.sql`, `Medium/Active Businesses.sql`, `Hard/Average Salary.sql`) to demonstrate the new documentation structure. These files now include placeholders for detailed explanations and key concepts. - **`CONTRIBUTING.md`:** Added a new file with guidelines for community contributions, referencing the `DOCUMENTATION_GUIDE.md`. - **`LICENSE`:** Added an MIT License file to clarify usage rights. This reorganization aims to make the repository more user-friendly for learning SQL and to provide a consistent framework for documenting solutions.
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‎CONTRIBUTING.md‎

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# Contributing to SQL LeetCode Tutorials
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Thank you for considering contributing to this repository! Your help is appreciated in making this a comprehensive resource for learning SQL through LeetCode problems.
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## How to Contribute
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We welcome contributions in various forms:
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* **Adding New Solutions:** If you've solved a LeetCode SQL problem that isn't here yet.
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* **Improving Explanations:** If you can provide a clearer or more detailed explanation for an existing solution.
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* **Adding Key Concepts:** If you can identify and list relevant SQL concepts for a solution.
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* **Fixing Errors:** If you find any errors in problem descriptions, solutions, or explanations.
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## Contribution Guidelines
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To ensure consistency and quality, please follow these guidelines:
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1. **Documentation Standard:** All contributions must adhere to the structure outlined in the `DOCUMENTATION_GUIDE.md`. Please familiarize yourself with it before submitting your changes.
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2. **File Naming:**
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* Use the official LeetCode problem title for the filename, replacing spaces with hyphens (e.g., `Easy/Find-Customers-With-Positive-Revenue-This-Year.sql`).
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* Place the file in the appropriate difficulty directory (`Easy/`, `Medium/`, `Hard/`).
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3. **Problem Information:**
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* Ensure the "Problem Title (from LeetCode)" and "LeetCode Link" sections are accurately filled out.
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4. **Clear Explanations:** If adding or modifying an explanation, make it clear, concise, and easy to understand. Break down complex logic.
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5. **Accurate Key Concepts:** List the primary SQL concepts demonstrated by the solution.
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6. **One Problem Per Pull Request:** If you're submitting multiple new solutions, please create a separate pull request for each problem. This makes reviewing easier.
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## Setting Up Your Environment (General Guide)
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1. **Fork the Repository:** Click the 'Fork' button at the top right of this page.
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2. **Clone Your Fork:** `git clone https://github.com/YOUR_USERNAME/SQL-Leetcode-Challenge.git`
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3. **Create a New Branch:** `git checkout -b your-feature-branch-name` (e.g., `add-solution-median-employee-salary`)
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4. **Make Your Changes:** Add your solution or improvements, ensuring you follow the `DOCUMENTATION_GUIDE.md`.
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5. **Commit Your Changes:** `git commit -am "feat: Add solution for Median Employee Salary"` (Follow conventional commit message standards if possible).
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6. **Push to Your Fork:** `git push origin your-feature-branch-name`
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7. **Open a Pull Request:** Go to the original repository page on GitHub and click 'New pull request'.
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## Code of Conduct
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While this is a small project, please be respectful and constructive in all communications.
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---
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Thank you for contributing!

‎DOCUMENTATION_GUIDE.md‎

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# Documentation Guide for LeetCode Problems
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This guide outlines the standard format for documenting SQL LeetCode problems in this repository. Adhering to this structure will ensure consistency and make it easier for users to learn.
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Each problem solution file should be a Markdown file (`.md`) or a SQL file (`.sql`) that includes Markdown-formatted comments at the top, and should contain the following sections:
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## Problem Title (from LeetCode)
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* Clearly state the official title of the LeetCode problem.
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## LeetCode Link
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* Provide a direct hyperlink to the problem on the LeetCode website.
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* Example: `[Problem Name](https://leetcode.com/problems/problem-name/)`
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## Problem Description
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* Copy and paste the complete problem description from LeetCode.
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* Ensure all formatting (like tables, examples, constraints) is preserved as much as possible using Markdown.
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## SQL Solution
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* Present the complete SQL solution.
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* Use SQL code blocks for proper formatting and syntax highlighting.
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```sql
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-- Your SQL solution here
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SELECT column_name
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FROM table_name
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WHERE condition;
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```
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## Explanation
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* This is a critical section for learning.
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* Provide a clear, step-by-step breakdown of your SQL solution.
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* Explain the thought process:
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* Why did you choose certain SQL clauses (e.g., `JOIN` vs. `LEFT JOIN`)?
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* How does the logic flow to arrive at the correct answer?
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* Break down complex queries into smaller, understandable parts.
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* Explain any specific functions or techniques used.
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* Use clear and concise language.
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## Key Concepts
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* List the main SQL concepts and techniques demonstrated in the solution.
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* This helps users identify what they are learning or can practice with this problem.
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* Examples:
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* `INNER JOIN`
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* `GROUP BY`
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* `HAVING`
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* `Window Functions (e.g., ROW_NUMBER(), RANK())`
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* `Common Table Expressions (CTEs)`
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* `Subqueries`
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* `CASE WHEN statements`
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* `Date/Time Functions`
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* `String Manipulation`
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---
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**Note:** If you are documenting a solution within a `.sql` file, please use comment blocks for each section heading, for example:
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```sql
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/*
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## Problem Title (from LeetCode)
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...
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*/
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/*
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## LeetCode Link
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...
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*/
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/*
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## Problem Description
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...
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*/
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/*
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## SQL Solution
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*/
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-- Your SQL solution here
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SELECT column_name
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FROM table_name
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WHERE condition;
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/*
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## Explanation
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...
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*/
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/*
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## Key Concepts
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...
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*/
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```
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By following this guide, we can create a valuable and consistent learning resource.

‎Easy/Ads performance.sql‎

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-- Question 13
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-- Table: Ads
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-- +---------------+---------+
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-- | Column Name | Type |
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-- +---------------+---------+
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-- | ad_id | int |
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-- | user_id | int |
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-- | action | enum |
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-- +---------------+---------+
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-- (ad_id, user_id) is the primary key for this table.
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-- Each row of this table contains the ID of an Ad, the ID of a user and the action taken by this user regarding this Ad.
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-- The action column is an ENUM type of ('Clicked', 'Viewed', 'Ignored').
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/*
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## Problem Title (from LeetCode)
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(Placeholder: Add LeetCode problem title here)
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*/
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/*
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## LeetCode Link
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(Placeholder: Add direct link to LeetCode problem here)
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*/
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/*
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## Problem Description
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Table: Ads
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+---------------+---------+
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| Column Name | Type |
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+---------------+---------+
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| ad_id | int |
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| user_id | int |
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| action | enum |
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+---------------+---------+
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(ad_id, user_id) is the primary key for this table.
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Each row of this table contains the ID of an Ad, the ID of a user and the action taken by this user regarding this Ad.
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The action column is an ENUM type of ('Clicked', 'Viewed', 'Ignored').
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-- A company is running Ads and wants to calculate the performance of each Ad.
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-- Performance of the Ad is measured using Click-Through Rate (CTR) where:
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-- Write an SQL query to find the ctr of each Ad.
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-- Round ctr to 2 decimal points. Order the result table by ctr in descending order and by ad_id in ascending order in case of a tie.
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-- The query result format is in the following example:
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-- Ads table:
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-- +-------+---------+---------+
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-- | ad_id | user_id | action |
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-- +-------+---------+---------+
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-- | 1 | 1 | Clicked |
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-- | 2 | 2 | Clicked |
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-- | 3 | 3 | Viewed |
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-- | 5 | 5 | Ignored |
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-- | 1 | 7 | Ignored |
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-- | 2 | 7 | Viewed |
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-- | 3 | 5 | Clicked |
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-- | 1 | 4 | Viewed |
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-- | 2 | 11 | Viewed |
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-- | 1 | 2 | Clicked |
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-- +-------+---------+---------+
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-- Result table:
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-- +-------+-------+
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-- | ad_id | ctr |
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-- +-------+-------+
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-- | 1 | 66.67 |
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-- | 3 | 50.00 |
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-- | 2 | 33.33 |
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-- | 5 | 0.00 |
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-- +-------+-------+
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-- for ad_id = 1, ctr = (2/(2+1)) * 100 = 66.67
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-- for ad_id = 2, ctr = (1/(1+2)) * 100 = 33.33
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-- for ad_id = 3, ctr = (1/(1+1)) * 100 = 50.00
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-- for ad_id = 5, ctr = 0.00, Note that ad_id = 5 has no clicks or views.
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-- Note that we don't care about Ignored Ads.
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-- Result table is ordered by the ctr. in case of a tie we order them by ad_id
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-- Solution
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A company is running Ads and wants to calculate the performance of each Ad.
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Performance of the Ad is measured using Click-Through Rate (CTR) where:
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Write an SQL query to find the ctr of each Ad.
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Round ctr to 2 decimal points. Order the result table by ctr in descending order and by ad_id in ascending order in case of a tie.
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The query result format is in the following example:
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Ads table:
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+-------+---------+---------+
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| ad_id | user_id | action |
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+-------+---------+---------+
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| 1 | 1 | Clicked |
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| 2 | 2 | Clicked |
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| 3 | 3 | Viewed |
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| 5 | 5 | Ignored |
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| 1 | 7 | Ignored |
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| 2 | 7 | Viewed |
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| 3 | 5 | Clicked |
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| 1 | 4 | Viewed |
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| 2 | 11 | Viewed |
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| 1 | 2 | Clicked |
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+-------+---------+---------+
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Result table:
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+-------+-------+
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| ad_id | ctr |
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+-------+-------+
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| 1 | 66.67 |
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| 3 | 50.00 |
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| 2 | 33.33 |
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| 5 | 0.00 |
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+-------+-------+
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for ad_id = 1, ctr = (2/(2+1)) * 100 = 66.67
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for ad_id = 2, ctr = (1/(1+2)) * 100 = 33.33
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for ad_id = 3, ctr = (1/(1+1)) * 100 = 50.00
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for ad_id = 5, ctr = 0.00, Note that ad_id = 5 has no clicks or views.
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Note that we don't care about Ignored Ads.
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Result table is ordered by the ctr. in case of a tie we order them by ad_id
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*/
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/*
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## SQL Solution
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*/
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with t1 as(
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select ad_id, sum(case when action in ('Clicked') then 1 else 0 end) as clicked
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from ads
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select *
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from t1 join t2
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on t1.ad_id = t2.ad) a
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order by ctr desc, ad_id
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order by ctr desc, ad_id
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/*
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## Explanation
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(Placeholder: Explanation to be added.)
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*/
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/*
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## Key Concepts
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(Placeholder: List key SQL concepts used, e.g., JOIN, GROUP BY, CTEs, etc.)
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*/

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