Add SQL examples for DENSE_RANK() usage

[theammarngp-makes]

This SQL script provides examples of using the DENSE_RANK() window function to assign ranks to employees based on their manager_id and emp_id. It includes multiple queries demonstrating various use cases and comparisons with the RANK() function.

📝 Summary

Closes #

🎯 Type of Change

• 🐛 Bug Fix - Fixes a SQL error or broken query in existing content
• ✨ New Content - Adds new SQL examples, sections, or topics
• 📖 Documentation - Improves explanations, clarity, or formatting
• 🔄 Refactor - Reorganizes or improves structure without changing content
• 🏗️ Schema/Database - Updates sample schema, test data, or database setup
• 🎓 Learning Resources - Adds references, resources, or practice exercises

📋 Changes Made

• Change 1
• Change 2
• Change 3

🧪 Testing Checklist

• ✅ All SQL queries have been tested and execute without errors
• ✅ Queries work on both MySQL 8.0+ and PostgreSQL (if applicable)
• ✅ Sample data matches the schema defined in 10_Schema/
• ✅ Query results are accurate and match the expected business logic
• ✅ No syntax errors or deprecated SQL syntax used
• ✅ Tested on different SQL client tools (Workbench, DBeaver, etc.) if relevant
• ✅ Performance considerations are documented (if applicable)
• ✅ Common mistakes and edge cases are explained

📚 Documentation Checklist

• ✅ Business context/use case is clearly explained
• ✅ SQL solution is documented with comments
• ✅ Explanation covers what the query does and why
• ✅ Common mistakes section is included (if applicable)
• ✅ Interview tips/follow-up questions are provided (if applicable)
• ✅ Practice challenges are included (if applicable)
• ✅ Links to related content are provided where relevant
• ✅ No broken links in documentation
• ✅ Proper markdown formatting and structure
• ✅ Spelling and grammar checked

🔧 SQL Style Checklist

• ✅ SQL follows ANSI-standard conventions
• ✅ Database-agnostic (MySQL/PostgreSQL compatible) or dialect clearly noted
• ✅ Queries use meaningful table/column aliases
• ✅ Indentation and formatting is consistent and readable
• ✅ Comments explain complex logic or non-obvious parts
• ✅ Best practices are demonstrated (no SELECT * in production code)
• ✅ Performance is considered (indexes, query execution order)
• ✅ NULL handling is explicit and correct
• ✅ No hardcoded values; uses parameterization where appropriate
• ✅ Follows the SQL style guide in CONTRIBUTING.md

👥 Contributor Checklist

• ✅ I have read the CONTRIBUTING.md guidelines
• ✅ I have read the CODE_OF_CONDUCT.md
• ✅ My changes follow the repository's SQL style and structure
• ✅ I have added/updated documentation for my changes
• ✅ I have tested my changes thoroughly
• ✅ I have resolved any conflicts with the main branch
• ✅ I have not introduced any breaking changes
• ✅ My commit messages are clear and descriptive
• ✅ This PR is ready for review
• ✅ I understand that my contribution may be edited or improved

🎓 Learning Path Impact

• Target Audience: Beginners / Intermediate / Advanced / All Levels
• Related Sections: e.g., 01_Fundamentals, 03_Joins, etc.
• Difficulty Level: Easy / Medium / Hard
• Learning Objectives Met: (List key SQL concepts covered)

📸 Visual Changes (Optional)

• No visual changes
• Documentation formatting changes: (describe)
• New diagrams or examples: (describe)
• Table/schema visualization updates: (describe)

🔗 Related Issues and PRs

Related to: #
Depends on: #
Closes: #

📝 Additional Notes

---

✨ Thank you!

Thank you for contributing to the SQL Engineering Handbook! Your effort helps make SQL learning better for everyone. 🙏

If you have questions or need guidance, feel free to ask in the PR comments. Happy to help refine your contribution!