Project: Customers in Go

Time to put the module together. The shop wants a small API for its customers. GET /customers lists them, and GET /customers/{id} shows one of them, with how many orders they placed and how much they spent. Your helpers queryAll and queryOne do the talking to the database.

Numbers worked out from tables: the order count and the money spent are not stored anywhere. Every request works them out from orders joined with order_items, with COUNT and SUM, so they are always up to date. Watch for two traps. A JOIN gives one row per item, so count orders with COUNT(DISTINCT ...). SUM of no rows is NULL, so wrap it in COALESCE, which gives the first of its values that isn't NULL. For example, here is a handler for the stock of one category:

app.HandleFunc("GET /categories/{name}/stock", func(w http.ResponseWriter, r *http.Request) {
    query := "SELECT COUNT(*) AS products, COALESCE(SUM(stock), 0) AS in_stock " +
        "FROM products WHERE category = ?"
    totals := queryOne(query, r.PathValue("name"))
    writeJSON(w, 200, totals)
})

A category with no products gets "in_stock": 0, not null. Notice the space at the end of the first piece: + glues the pieces exactly as they are, and without it the SQL would read in_stockFROM. Your two handlers must answer:

GET /customers     →  200  [{"id": 1, "name": "Ana Pop", "city": "Cluj"}, {"id": 2, "name": "Ben Ortiz", "city": "Madrid"}, …, {"id": 5, "name": "Eva Rossi", "city": "Rome"}]
GET /customers/1   →  200  {"id": 1, "name": "Ana Pop", "email": "ana@example.com", "city": "Cluj", "joined": "2026-01-10", "orders": 2, "spent": 75}
GET /customers/2   →  200  {"id": 2, "name": "Ben Ortiz", …, "orders": 1, "spent": 29}
GET /customers/3   →  200  {"id": 3, "name": "Chloe Ng", …, "orders": 2, "spent": 89}
GET /customers/5   →  200  {"id": 5, "name": "Eva Rossi", …, "orders": 0, "spent": 0}
GET /customers/99  →  404  {"error": "Customer not found"}
POST /orders {"customer_id": 5, "items": [{"product_id": 1, "quantity": 2}, {"product_id": 4, "quantity": 3}]}  →  201  {"id": 8}
GET /customers/5   →  200  {"id": 5, "name": "Eva Rossi", …, "orders": 1, "spent": 30}

Your task: the two handlers are waiting at the end of newApp; fill them in. GET /customers answers the id, name and city of every customer. GET /customers/{id} answers the whole customer plus orders and spent (cancelled orders don't count), or a 404 when no customer has that id.

Assignment
Follow the Coding Tutorial to build the two customer handlers on top of queryAll and queryOne.

Hint
Read the customer with queryOne and check for nil first: answer 404 and return. The totals also come back as one row, because COUNT and SUM turn all the rows into one. Copy its "orders" and "spent" into the customer map before the last writeJSON.