docs: Add essential NULL handling functions (IFNULL and COALESCE)

[Navodhya-Fernando]

🎯 Add NULL Handling Functions to Essential MySQL Functions Guide

📋 Description

This PR adds a new section on NULL Handling Functions to the Essential MySQL Functions guide, covering two critical MySQL functions that were missing from the documentation:

• IFNULL() - Provides default values for NULL columns
• COALESCE() - Returns the first non-NULL value from multiple expressions

🔧 What's Added

New Section: "NULL Handling Functions" with clear explanations
Practical Examples: Real-world SQL code snippets with expected outputs
Use Cases: Employee bonus handling, player nickname management, and default value scenarios
Consistent Formatting: Follows the existing documentation style and structure

📚 Why This Matters

NULL handling is fundamental in SQL development, and these functions are essential for:

• Preventing unexpected NULL values in query results
• Providing fallback values when data is missing
• Creating robust queries that handle incomplete datasets
• Following SQL best practices for data integrity

🎃 Hacktoberfest 2025

This contribution enhances the learning experience for SQL beginners by filling a genuine gap in the educational content. The added functions are commonly used in production environments and essential for any comprehensive SQL guide.

✅ Checklist

• Content follows existing markdown formatting
• Code examples include proper syntax and table references
• Functions are explained with clear, practical use cases
• Output examples are provided for better understanding
• Section integrates seamlessly with existing content structure
• Added content to 018-essential-mysql-functions.md

[Copilot]

Pull Request Overview

This PR enhances the Essential MySQL Functions guide by adding a new section dedicated to NULL handling functions. The addition fills a gap in the documentation by covering two fundamental MySQL functions that are crucial for handling missing data in SQL queries.

Key changes:

• Added a comprehensive "NULL Handling Functions" section
• Documented IFNULL() and COALESCE() functions with practical examples
• Included multiple code examples showing real-world usage scenarios

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[Copilot]

Pull Request Overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated 2 comments.

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