Top 5 Categories by Month

Citizant's federal sales reporting dataset contains orders, line items, products, and product categories. Write a PostgreSQL query that identifies the five highest-selling categories within each calendar month.

Requirements

1. Include only orders whose status is completed.
2. Calculate sales as quantity * unit_price at the line-item level, then aggregate by month and category.
3. Rank categories independently within each month using a window function.
4. Return no more than five categories per month, ordered by month and descending sales rank.
5. Exclude products that do not map to a category, and handle nullable quantities correctly.