select cust_id,
    sum(item_price*quantity)  AS total_ordered
FROM Orders a, OrderItems b 
where a.order_num =b.order_num
group by cust_id
order by  total_ordered DESC;



select cust_id,
(select sum(quantity * item_price) 
    from  OrderItems
    where OrderItems.order_num=Orders.order_num
) AS total_ordered
from Orders 
order by total_ordered DESC;




# 方法一：最简单的方法
select cust_id, sum(quantity*item_price) as total_ordered 
from Orders join OrderItems using(order_num)
group by cust_id #注意要group否则有歧义
order by total_ordered desc;


# 方法二：先选列,再利用子查询选子列
select cust_id, 
# 从列里自定义子查询语句
(
    select sum(quantity*item_price)
    from OrderItems oi
    where oi.order_num = o.order_num
) as total_ordered
from Orders o
order by total_ordered desc;


# 方法三：从表与子表（子查询生成的表）找
select cust_id, total_ordered 
from Orders a,
(
    select order_num, sum(quantity*item_price) total_ordered   
    from OrderItems
    group by order_num #注意生成子表时需要group
) as t
where t.order_num = a.order_num
order by total_ordered desc;

# 方法四：表与子表联结(join)
select cust_id, total_ordered from Orders a
left join 
(
    select order_num, sum(quantity*item_price) as total_ordered
    from OrderItems
    group by order_num #注意生成子表时需要group
) as b
on a.order_num = b.order_num
order by total_ordered desc;