第一种方法：group by分组，having过滤

select
    order_num,
    sum(quantity*item_price) as total_price
from
    OrderItems
group by
    order_num
having
    total_price >= 1000
order by
   order_num asc

第二种方法：嵌套再嵌套（平常脑袋坏了才会这么写）

不过可以练着玩

select
    s.order_num,
    s.sumprice as total_price
from
    (select
        p.order_num as order_num,
        sum(p.price) as sumprice
    from
        (select
            order_num,
            quantity*item_price as price
        from
            OrderItems
        ) p
    group by
        p.order_num
     )s
where 
    s.sumprice >= 1000
order by
    s.order_num asc