MySQL加入4个表显示错误的总和

Question

我已经重写了我的要求。抱歉，不是具体的。你可以在这里找到小提琴：http://sqlfiddle.com/#!9/b1cbf1/2

我想输出：

# invoice 1 - customer 1, items total:500, quantity:5, paid 250
# invoice 2 - customer 1, items total: 360, quantity:4, paid 300
# invoice 3 - customer 2, items total - 650, quantity: 5, paid 300
# invoice 4 - customer 3, items total - 210, quantity: 2, paid 110
# invoice 5 - customer 3, items total - 0, quantity: 0, paid 0

我试过了：

SELECT 
    invoices.invoice_id,
    customers.last_name,
    COUNT(items.item_invoice_id) as quantity,
    SUM(payments.payment_amount) as total_paid,
    SUM(items.item_price) as subtotal

FROM `invoices` 
LEFT JOIN payments
     ON payments.invoice_id = invoices.invoice_id
LEFT JOIN customers
      ON customers.id = invoices.invoice_customer_id

LEFT JOIN items
       ON items.item_invoice_id = invoices.invoice_id

GROUP BY invoices.invoice_id