A company is implementing a new database system to manage its customer information. The database contains tables for customers, orders, products and sales representatives. Each customer can place multiple orders and each order can contain multiple products. Each order is associated with a specific sales representative who manages the customer account. The system keeps records of customers, their orders, the products included in each order, and the sales representatives responsible for handling the accounts. The database is designed to improve efficiency in tracking sales and customer interactions.

Which SQL query retrieves the total number of distinct customers who have placed orders?