Fixed Serial Numbers and Added a New SQL Question for Practice.

[sahil-soni18]

This PR addresses the following changes:

• Fixed the serial numbering issue in the existing SQL questions to ensure consistency across the repository.
• Added a new SQL question to enhance the learning experience.

Motivation and Context
The goal of this contribution is to ensure a consistent format for serial numbers in the questions and provide a new challenge for learners.

Checklist

• Serial numbers in the questions have been fixed and are now consistent.
• A new SQL question has been added with a detailed solution.