4. (2 points) Rank each customer by the number of orders which they’ve placed. The customer who placed most orders should be ranked number 1. Be sure to include customers that haven’t placed any orders yet. (HINT: Use RANK)

Assuming we have a table named "customers" with columns "customer_id" and "order_count", where "order_count" is the number of orders placed by each customer, we can use the following SQL query to rank the customers by the number of orders:

SELECT customer_id, order_count, RANK() OVER (ORDER BY order_count DESC) as rank
FROM customers

This query will return a result set with three columns: "customer_id", "order_count", and "rank". The "rank" column will contain the ranking of each customer based on their "order_count", with the customer who placed the most orders being ranked number 1. Customers who haven't placed any orders yet will also be included in the result set, with an "order_count" of 0 and a corresponding rank.