Question

假设我有三个表 - Orders，OrderDetails和ProductType - Orders表包含Customer列。我需要做的是编写一个查询，向我显示一个客户列表和每个客户放置了多少订单，以及显示和分组另一列，这是一个基于特定类型产品的布尔值 - 说，电话 - 在订单中。

例如，我们可能有：

Customer | NumOrders | IncludesPhone --------------------------------- Jameson | 3 | Yes Smith | 5 | Yes Weber | 1 | Yes Adams | 2 | | No Jameson | 1 | No Smith | 7 | No Weber | 2 | No

但是，当我尝试为此编写查询时，我将获得具有相同Customer和IncludesPhone值的多行，每个行具有不同的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

将组更改为

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

Answer 2

此查询应该有效

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

由于您要对Customer和Type进行分组，因此您的查询会返回每个类型的每个客户的订单数。如果您只需要每个客户一行，则应该只按Customer分组，然后使用类似的内容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

内部金额基本上计算每位客户购买的手机数量。如果这大于0，那么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列进行分组，因此可能存在重复的行。例如，类型“电子邮件”和“个人”列IncludesPhone将为“否”，但当您按Type分组时，输出中会有两条记录。

要解决此问题，您可以在group by子句中使用相同的表达式，也可以使用子查询或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

您可以尝试此查询：

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

组不按预期运行

5 个答案: