What an Order Contains

An order can hold several products. They are in a third table, order_items: one row per product in an order, with the order_id it belongs to, the product_id, the quantity and the unit_price, the price of one item when the order was placed. A JOIN with products gives each item its name.

Such a query can find many rows, and .all() gives them all as an array; as tells TypeScript the type of its rows. To add up a value over the rows, start a variable at 0 before a loop and add to it inside the loop. Here is the idea on another value, the prices of the kitchen products:

const rows = db.prepare("SELECT price FROM products WHERE category = ?").all("kitchen") as { price: number }[];
let total = 0;
for (const row of rows) {
  total = total + row.price;
}

An order's total uses unit_price, the price saved with the order, not the product's price today: when the shop changes a price, the orders already placed keep their totals. Every order has at least one item, so an empty array of rows means there is no such order.

Your route must give these answers (the PUT raises the Mug's price to 15 before order 7 is read):

GET /orders/1/items   →  200  {"order_id": 1, "items": [{"name": "Mug", "quantity": 2, "unit_price": 12}, {"name": "Pen", "quantity": 5, "unit_price": 2}], "total": 34}
GET /orders/5/items   →  200  {"order_id": 5, "items": [{"name": "Teapot", "quantity": 2, "unit_price": 22}, {"name": "Lamp", "quantity": 1, "unit_price": 29}], "total": 73}
GET /orders/6/items   →  200  {"order_id": 6, "items": [{"name": "Stapler", "quantity": 1, "unit_price": 9}], "total": 9}
PUT /products/1  {"name": "Mug", "price": 15, "category": "kitchen"}  →  200  {"id": 1, "name": "Mug", "price": 15, "category": "kitchen", "stock": 10}
GET /orders/7/items   →  200  {"order_id": 7, "items": [{"name": "Mug", "quantity": 4, "unit_price": 12}, {"name": "Notebook", "quantity": 2, "unit_price": 7}], "total": 62}
GET /orders/99/items  →  404  {"error": "Order not found"}

Your task: add GET /orders/:orderId/items. It reads the order's items with their product names, adds up the total from the saved prices, and answers 404 when no item belongs to that order.

Assignment
Follow the Coding Tutorial to add the route /orders/:orderId/items, its JOIN query and the loop that builds the items and the total.

Hint
.all() never gives undefined: when nothing matches, it gives an empty array, so check rows.length === 0. Declare total with let and start it at 0 before the loop, not inside it.