sql - SQL - 删除重复项以显示最新的日期记录

Question

我有一个观点，最终我想为每个客户返回 1 行。

目前它的选择如下；

SELECT 
    Customerid,
    MAX(purchasedate) AS purchasedate,
    paymenttype,
    delivery,
    amount,
    discountrate 
FROM    
    Customer 
GROUP BY
    Customerid,
    paymenttype,
    delivery,
    amount,
    discountrate

我希望 MAX(purchasedate) 可以工作，但是当我进行分组时，它会中断，因为有时可能会有折扣，有时它是 NULL，每个客户的付款类型也可能不同，是否只是为了显示客户的最后一次购买?

Answer 1

由于SQL Server 2008 r2支持windows功能，

SELECT  Customerid,
        purchasedate,
        paymenttype,
        delivery,
        amount,
        discountrate
FROM    
    (
        SELECT  Customerid,
                purchasedate,
                paymenttype,
                delivery,
                amount,
                discountrate,
                ROW_NUMBER() OVER (Partition By CustomerID
                                    ORDER BY purchasedate DESC) rn
        FROM    Customer
    ) derivedTable
WHERE   derivedTable.rn = 1

或通过使用Common Table Expression

WITH derivedTable
AS
(
    SELECT  Customerid,
            purchasedate,
            paymenttype,
            delivery,
            amount,
            discountrate,
            ROW_NUMBER() OVER (Partition By CustomerID
                                ORDER BY purchasedate DESC) rn
    FROM    Customer
)
SELECT  Customerid,
        purchasedate,
        paymenttype,
        delivery,
        amount,
        discountrate
FROM    derivedTable
WHERE   derivedTable.rn = 1

或者通过在其他 DBMS 中使用子查询的连接

SELECT  a.*
FROM    Customer a
        INNER JOIN
        (
            SELECT  CustomerID, MAX(purchasedate) maxDate
            FROM    Customer
            GROUP BY CustomerID
        ) b ON  a.CustomerID = b.CustomerID AND
                a.purchasedate = b.maxDate