SELECT 
    c.cust_name cust_name,
    tb.total_price total_price
FROM
    Customers c INNER JOIN Orders o ON
    c.cust_id = o.cust_id INNER JOIN (
        SELECT
            order_num,
            SUM(item_price * quantity) total_price
        FROM
            OrderItems
        GROUP BY
            order_num
        HAVING
            total_price >= 1000
    ) tb ON
    o.order_num = tb.order_num
ORDER BY
    tb.total_price;