A customer wants sales reports grouped by month and region. How would you write an efficient SQL query?

I am preparing for SQL and database interviews and came across this real-world reporting scenario. Suppose a customer wants a sales report that summarizes total sales grouped by month and region from a large sales database.

I would like to understand:

How would you write an efficient SQL query to generate this report?
Which SQL functions should be used to group records by month and region?
How can aggregate functions such as SUM(), COUNT(), and AVG() be used in this scenario?
What indexes would you recommend to improve query performance on large datasets?
How would you handle missing or NULL values in the report?
Could you provide a sample SQL query along with an explanation of how it works?
Are there any best practices for optimizing reporting queries on tables containing millions of records?