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 db.queryForList gives them all as a list. 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:

List<Map<String, Object>> rows = db.queryForList("SELECT price FROM products WHERE category = ?", "kitchen");
int total = 0;
for (Map<String, Object> row : rows) {
    total = total + (int) row.get("price");
}

row.get("price") gives an Object, so (int) in front of it makes it a number you can add. 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 list 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 method getOrderItems, its JOIN query and the loop that builds the items and the total.

Hint
queryForList never gives null: when nothing matches, it gives an empty list, so check rows.isEmpty(). Start total at 0 before the loop, not inside it. Java won't compile the method until it has a return after the if.