Question

所以，我有这个查询

select C.custID, C.custName, count(C.custID) as totalOrder from Customer C, Purchase P
where C.custID = P.custID and P.purchaseDate between '2015-01-01' and '2015-12-31'
group by C.custID, C.custName

此查询显示Customers的所有记录。如何修改此查询以便它只显示具有最高totalOrder的Customer记录？

我可以尝试使用having count(C.custID) = ...，但后来我不知道还能做什么

Answer 1

您可以使用子选择来执行此操作（以及JOIN以进一步优化它。）

另外，希望我不会太讨厌，但我不确定你应该GROUP BY客户ID和名字。 ID应该足够了。如果客户可以拥有相同的ID和不同的名称，则两者分组将起作用。与ID 1, Name John和ID 1, Name Jeff一样。如果确实如此，它可能不应该是这样的：P

无论如何，我建议你试试这个：

SELECT total_orders.custID, total_orders.custName, MAX(total_orders.totalOrder) FROM (SELECT c.custID, c.custName, count(c.custID) as totalOrder FROM Customer c LEFT JOIN Purchase p ON c.custID = p.custID WHERE p.purchaseDate BETWEEN '2015-01-01' AND '2015-12-31' GROUP BY c.custID, c.custName) as total_orders

首先，您选择和计算与之前相同的内容，但将其存储为一种虚拟表（执行SELECT ... FROM (<another select>) as something something是虚拟表< / em>），从中选择MAX(totalOrder)。

Answer 2

解决。我认为它是这样的：

select C.custID, C.custName, count(C.custID) as totalOrder from Customer C, Purchase P
where C.custID = P.custID and P.purchaseDate between '2015-01-01' and '2015-12-31'
group by C.custID, C.custName having count(C.custID) = 
(select max(t.totalOrder) from 
(select C.custID, C.custName, count(C.custID) as totalOrder from Customer C, Purchase P
where C.custID = P.custID and P.purchaseDate between '2015-01-01' and '2015-12-31'
group by C.custID, C.custName) t)

请注意我已添加

having count(C.custID) = 
(select max(t.totalOrder) from 
(select C.custID, C.custName, count(C.custID) as totalOrder from Customer C, Purchase P
where C.custID = P.custID and P.purchaseDate between '2015-01-01' and '2015-12-31'
group by C.custID, C.custName) t)

所以它会找到totalOrder的最大值。并将count(C.custID)分配给totalOrder的最大值。这可能是一个非常糟糕的代码，但它的工作原理。

如果有人有更简单的方法来解决这个问题，请分享代码。

选择表上的最高值

2 个答案: