Oracle SQL 使用子查询

Question

我在学习子查询时遇到了一些问题。我正在尝试显示每个客户订购过的最昂贵的书，并使用不相关的子查询来实现。

这是我目前的情况：

select Firstname || ', '|| Lastname "Name", title, retail
from Customers join orders using (Customer#) join orderitems using (Order#)                 
join books using (Isbn)
where retail =ANY (select max(retail)
                from books join orderitems using (isbn) join Orders using (order#) join Customers using (Customer#)
                group by Customer#)                 
order by  Firstname, Lastname;

内部查询显示每个客户的最大零售额，但我不确定为什么外部查询会多次显示客户。我刚刚开始学习 SQL，如有任何帮助，我们将不胜感激

Answer 1

select 
  Firstname || ', '|| Lastname "Name"
  ,max(retail) "Most expensive book by customer"
from 
  Customers c, Orders o, OrderItems oi, Books b 
where c.customer#=o.customer# 
  and o.order#=oi.order# 
  and oi.isbn=b.isbn
group by Firstname || ', '|| Lastname;

Answer 2

这里的缺陷是你的逻辑，而不是语法。内部查询 return 是每个客户的最高零售价，然后外部查询 return 是与这些价格之一匹配的任何连接行。考虑您有两个客户的情况 - 客户 1 进行了购买，一位购买了 10 美元，一位购买了 20 美元，而客户 2 只购买了一次 10 美元的商品。内部查询将 return 20$（客户 1 的最大购买量）和 10$（客户 2 的最大购买量），然后外部查询将 return all 这些金额中的任何一个的购买，即所有三行。 "textbook" 解决方案是 rank 每个客户的购买，并且 return 每个客户只有前一个：

SELECT   name, title, retail
FROM     (SELECT firstname || ', '|| lastname AS name,
                 title, 
                 retail,
                 RANK() OVER (PARTITION BY customer# ORDER BY retail DESC) AS rk
          FROM   customers
          JOIN   orders USING (customer#)
          JOIN   orderitems USING (order#)                 
          JOIN   books USING (isbn))
WHERE    rk = 1
ORDER BY 1

Answer 3

select Firstname || ', '|| Lastname "Name", title, retail
from Customers join orders using (Customer#) join orderitems using (Order#)                 
join books using (Isbn)
where (Customer#, retail) in (select Customer#,  max(retail)
                from books join orderitems using (isbn) join Orders using (order#) join Customers using (Customer#)
                group by Customer#)                 
order by  Firstname, Lastname;

或者我喜欢的方式，无需选择相同的东西两次：

select Name, title, Customer#, retail
from (select Firstname || ', '|| Lastname "Name", title, Customer#,  
             retail, max(retail) over (partition by  Customer#) max_retail
        from books 
        join orderitems using (isbn) 
        join Orders using (order#) 
        join Customers using (Customer#))    
where retail=max_retail             
order by  Firstname, Lastname;

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