SQL 查询使用连接查找最大值

Question

我正在尝试解决这个问题 - 根据给定的数据集，编写 SQL 查询以查找 1996 年下订单数量最多的客户的 CustomerID。

这是我写的，但这似乎没有给出正确答案 -

select c.customerId
     , COUNT(*) 
  from orders o 
  JOIN customers c 
    ON o.customerId = c.customerId 
 WHERE YEAR(o.orderDate  ) = 1996 
 GROUP 
    BY c.customerId

Answer 1

这应该可行（下面的逻辑使用 windows 函数）-

 Select t.customerId
 from
     (Select Tab.*,
            Rank() over(partition by customerId order by number_of_orders desc) as rank_orders
            from
             (select    c.customerId,
                     , COUNT(distinct order_id)  As number_of_orders
                       from 
                       orders as o 
                       INNER JOIN 
                       customers as c 
                       ON o.customerId = c.customerId 
                       WHERE YEAR(o.orderDate ) = 1996 
                       group by c.customerId) Tab) t
   where t.rank_orders = 1;

Answer 2

您可以将 window 函数与 group by 一起使用：

select customerid
from (select o.customerId, count(*),
             rank() over (order by count(*) desc) as seqnum
      from orders o
      where o.orderDate >= '1996-01-01' and o.orderDate < '1997-01-01'
      group by o.customerId
    ) c
where seqnum = 1;

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