Create comprehensive repository structure for solving daily SQL LeetCode problems

[Copilot]

This PR transforms the basic repository into a fully-structured platform for organizing and solving SQL LeetCode problems on a daily basis.

What's New

📁 Organized Directory Structure

• Easy/, Medium/, Hard/ folders for difficulty-based organization
• Database-Design/ for schema and optimization problems
• Schemas/ with common table structures used across LeetCode problems
• Templates/ containing solution templates and formatting guidelines

📖 Comprehensive Documentation

• Enhanced README.md with clear structure explanation, progress tracking, and usage instructions
• CONTRIBUTING.md with detailed submission guidelines and quality standards
• Individual README files for each difficulty level explaining problem characteristics and study tips

🛠️ Developer Tools

• solution_template.sql providing consistent formatting for all solutions
• common_schemas.sql with pre-built table structures (Employee, Customer, Orders, etc.)
• setup.sh script for automated local environment initialization
• Proper .gitignore to exclude build artifacts and temporary files

🎯 Working Examples

Two complete example solutions demonstrate the repository structure:

Easy Problem - 175 Combine Two Tables:

SELECT 
    p.FirstName,
    p.LastName,
    a.City,
    a.State
FROM 
    Person p
    LEFT JOIN Address a ON p.PersonId = a.PersonId;

Medium Problem - 178 Rank Scores:

SELECT 
    score,
    DENSE_RANK() OVER (ORDER BY score DESC) as rank
FROM 
    Scores
ORDER BY 
    score DESC;

Key Features

🚀 Ready for Daily Use

• Clear naming convention: {ProblemNumber}_{ProblemName}/
• Consistent solution format with problem statements, schemas, and explanations
• Progress tracking system in main README

📚 Educational Value

• Difficulty-specific guidance and problem categorization
• Common SQL patterns documentation (JOINs, Window Functions, CTEs)
• Time/space complexity analysis in solutions

🤝 Contribution-Friendly

• Detailed contribution guidelines with code style requirements
• Template-based approach ensuring consistency
• Quality standards for documentation and code

The repository now provides a complete framework for the stated goal of "solving daily SQL Leetcode questions" with proper organization, documentation, and tooling to support both individual learning and community contributions.

Original prompt

Solving Daily SQL Leetcode questions

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