public class PurchaseOrder{
    @Id
    int id;
    int quantity;
    @ManyToOne
    Product product;
    @ManyToOne
    Customer customer;
}

public class Product{
    @Id
    int id;
    double price;
    String name;
    @OneToMany
    List<PurchaseOrder> purchaseOrderList
}

select p.ID, p."NAME", sum(p.PRICE * po.QUANTITY) as total from product p
join purchase_order po on p.ID = po.PRODUCT group by p.ID, p."NAME" order by total

EntityManager em = fb.nativeEntityManagerFactory.createEntityManager();
JPAQuery query = new JPAQuery(em);
QPurchaseOrder qp = QPurchaseOrder.purchaseOrder;
NumberExpression total = qp.quantity.multiply(qp.product.price);

List<Tuple> l = query.from(qp).groupBy(qp.product.id, qp.product.name).
orderBy(total.desc()).list(total.sum(), qp.id.count(), qp.product.id, qp.product.name);