题解 | #返回每个顾客不同订单的总金额#

整理了一下 子查询的方法，其中方法三、四很像，有点重复。

感觉题目给的示例是错误的，cust2不应该有两行，所以很多子查询(从OrderItems里)形成子表的时候group by order_num完了都应该是只有一个cust2的，有一个评论说如果想要两个cust2就需要在方法一里改成group by cust_id, order_num。

还有我很疑惑MySQL的注释不是--吗为啥这里是#😂

# 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;