Write SQL and merge linked lists

The technical interview included two coding-style tasks: 1. **SQL analytics task** You are given three tables: - `customers(customer_id, customer_name)` - `orders(order_id, customer_id, order_date, amount)` - `returns(return_id, order_id, created_at)` Write a SQL query that returns one row per customer, including customers who have never placed an order. For each customer, report: - `total_orders` - `total_amount` - `latest_order_date` - `latest_order_amount` Use a `LEFT JOIN` so customers with no orders still appear. Use a nested query, common table expression, or window function to identify the most recent order for each customer. 2. **List manipulation task** Given the heads of two singly linked lists whose values are already sorted in non-decreasing order, merge them into one sorted linked list by reusing the existing nodes, and return the head of the merged list.