分组方式未按预期运行

Question

假设我有三个表——Orders、OrderDetails 和 ProductType——并且 Orders 表包含一个 Customer 列。我需要做的是编写一个查询，它将向我显示客户列表以及每个客户下了多少订单，以及按另一列显示和分组，这是一个基于特定类型产品的布尔值 - 比如说, 电话 - 按顺序排列。

例如，我们可能有：

客户 |订单数量 |包括电话
---------------------------------
詹姆逊 | 3 |是的
史密斯 | 5 |是的
韦伯 | 1 |是的
亚当斯 | 2 | |没有
詹姆森 | 1 |没有
史密斯 | 7 |没有
韦伯 | 2 |没有

但是，当我尝试为此编写查询时，我得到了多行具有相同的 Customer 和 IncludesPhone 值，每行具有不同的 NumOrders 值。为什么会这样？我的查询如下：

SELECT Customer, COUNT(Customer) AS NumOrders, CASE WHEN (ProductType.Type = 'Phone') THEN 'Yes' ELSE 'No' END AS IncludesPhone
FROM Orders INNER JOIN OrderDetails INNER JOIN ProductType
GROUP BY Customer, Type
Order By IncludesPhone, Customer

Answer 1

将分组改为

GROUP BY    Customer, 
            CASE WHEN (ProductType.Type = 'Phone') THEN 'Yes' ELSE 'No' END

Answer 2

这个查询应该可以工作

SELECT Customer, COUNT(Customer) AS NumOrders, 
CASE WHEN (ProductType.Type = 'Phone') THEN 'Yes' ELSE 'No' END AS IncludesPhone
FROM Orders INNER JOIN OrderDetails INNER JOIN ProductType
GROUP BY    Customer, 
CASE WHEN (ProductType.Type = 'Phone') THEN 'Yes' ELSE 'No' END
Order By IncludesPhone, Customer

Answer 3

由于您同时对Customer 和Type 进行分组，因此您的查询会返回每种类型的每位客户的订单数。如果您只希望每个客户一行，您应该只按Customer 分组，然后使用类似这样的方法来确定给定客户是否购买了手机：

SELECT Customer, COUNT(Customer) AS NumOrders, 
    CASE 
        WHEN SUM(CASE WHEN (ProductType.Type = 'Phone') THEN 1 ELSE 0 END) > 0 
        THEN 'Yes' 
    ELSE 'No' END AS IncludesPhone
FROM Orders INNER JOIN OrderDetails INNER JOIN ProductType
GROUP BY Customer
Order By IncludesPhone, Customer

内部总和基本上计算每个客户购买的手机数量。如果大于 0，则客户至少购买了一部手机，我们返回“是”。

Answer 4

那是因为您按Type 列分组，所以可能有重复的行。例如，对于类型“电子邮件”和“个人”列，IncludesPhone 将为“否”，但当您按 Type 分组时，输出中将有两条记录。

要解决此问题，您可以在 group by 子句中使用相同的表达式或使用子查询或 Common Table Expression：

with cte as (
    select
        Customer,
        case when pt.Type = 'Phone' then 'Yes' else 'No' end as IncludesPhone
    from Orders as o
        inner join OrderDetails as od -- ???
        inner join ProductType as pt -- ???
)
select Customer, IncludesPhone, count(*) as NumOrders
from cte
group by Customer, IncludesPhone
order by IncludesPhone, Customer

在group by 子句中使用相同的表达式：

select
    Customer,
    case when pt.Type = 'Phone' then 'Yes' else 'No' end as IncludesPhone,
    count(*) as NumOrders
from Orders as o
    inner join OrderDetails as od -- ???
    inner join ProductType as pt -- ???
group by Customer, case when pt.Type = 'Phone' then 'Yes' else 'No' end

Answer 5

你可以试试这个查询：

SELECT  x.Customer,x.NumOrders,
        CASE WHEN x.NumOrders>0 AND EXISTS(
            SELECT  *
            FROM    Orders o
            INNER JOIN OrderDetails od ON ...
            INNER JOIN ProductType pt ON ...
            WHERE   o.Customer=x.Customer
            AND pt.Type = 'Phone'
        ) THEN 1 ELSE 0 END IncludesPhone
FROM
(
    SELECT  Customer,COUNT(Customer) AS NumOrders
    FROM    Orders 
    GROUP BY Customer
) x
Order By IncludesPhone, x.Customer;

或者这个：

SELECT  o.Customer, 
        COUNT(o.Customer) AS NumOrders, 
        MAX(CASE WHEN EXISTS
        (
            SELECT  *
            FROM    OrderDetails od
            JOIN    ProductType pt ON ...
            WHERE   o.OrderID=od.OrderID -- Join predicated between Orders and OrderDetails table
            AND     ProductType.Type = 'Phone'
        ) THEN 1 ELSE 0 END) AS IncludesPhone
FROM    Orders o
GROUP BY Customer
ORDER BY IncludesPhone, o.Customer