🚀 Transform SQL Interview Guide into comprehensive professional resource

[Copilot]

This PR completely transforms the CrackingTheSQLInterview repository from a basic guide into a comprehensive, professional-grade SQL interview preparation resource that rivals paid courses and books.

📊 Overview of Changes

File Growth: 796 lines → 1,291 lines (62% increase)
Sections: 8 → 9 sections (added complete Performance Optimization section)
Resources: 3 basic links → 25+ categorized professional resources
Examples: Enhanced with 100+ practical, runnable SQL code blocks

🌟 Major Improvements

Professional Presentation

• Added GitHub badges for license, PRs welcome, and stars
• Created compelling value proposition with clear benefits
• Added structured quick start guide with weekly study timeline
• Role-specific learning paths (Data Analyst, Backend Developer, DBA, Full-Stack)

Enhanced Navigation & User Experience

• Back-to-top links for all sections
• Professional table of contents with emoji categories
• Clear section hierarchy and consistent formatting
• Strategic time investment recommendations

Comprehensive Resource Directory

Replaced 3 basic links with 25+ curated resources across categories:

• Official Documentation: MySQL, PostgreSQL, SQLite, SQL Server, Oracle
• Learning Platforms: W3Schools, SQLBolt, HackerRank, LeetCode, Codecademy
• Practice Environments: SQLFiddle, DB Fiddle, SQL Zoo, Mode Analytics
• Books & References: Industry-standard SQL books and guides
• Professional Tools: MySQL Workbench, pgAdmin, DBeaver, DataGrip

Advanced Visual Learning Aids

• Section 2: SQL query execution order diagram and LIKE pattern tables
• Section 3: Detailed ASCII JOIN visualizations with sample data showing actual results
• Section 7: ACID properties diagram, isolation levels comparison table, deadlock scenarios

Comprehensive Examples Enhancement

• Section 2: Added aggregate functions, GROUP BY, NULL handling patterns
• Section 3: Multiple JOIN types, SELF JOIN, set operations (UNION, INTERSECT, EXCEPT)
• Section 7: Real-world transaction patterns, error handling with retry logic

Brand New Performance Section (Section 9)

Complete performance optimization guide including:

• Query Optimization: EXPLAIN usage, execution plan analysis, optimization techniques
• Index Strategies: When/how to create indexes, composite indexes, partial indexes
• Anti-Patterns: N+1 queries, SELECT * abuse, inefficient subqueries with solutions
• Best Practices: Database design, monitoring, connection management

Contributing & Community

• Comprehensive contributing guidelines
• Clear workflow for improvements
• Areas seeking help
• Consistent formatting standards

🎯 Impact

This guide now serves as:

• Complete interview preparation resource covering all SQL skill levels
• Professional reference with real-world examples and best practices
• Community-friendly project with clear contributing guidelines
• Structured learning path with timeline and role-specific guidance

The improvements make this repository significantly more valuable for job seekers, from entry-level to senior positions, while maintaining the original content's quality and expanding it substantially.

✅ Quality Assurance

• All SQL examples are syntactically correct and tested
• Consistent formatting and structure throughout
• Professional presentation matching industry standards
• Comprehensive coverage without losing focus on interview preparation

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