SELECT IFNULL(customer.cust_name, 'N/A') as customer_name
, IFNULL(product.prod_name, 'N/A') as product_name
, IFNULL(invoice_item.quantity, 0) as quantity
FROM ((customer
LEFT OUTER JOIN invoice ON customer.id = invoice.cust_id)
LEFT OUTER JOIN invoice_item ON invoice_item.invoice_id = invoice.id)
LEFT OUTER JOIN product ON invoice_item.product_id = product.id
ORDER BY customer.id, product.id;