Question

我在这里有一个问题，我不理解有关该主题的帖子。所以这是我的问题：

在我的申请中，我有以下实体：

Customer( CustomerId int, Name varchar(200), Email varchar(100), ...)
Employee (EmployeeId int, Name varchar(200), Email varchar(100), ...)
Sale(SaleId int, CustomerId int, EmployeeId int, SaleDate datetime, ...) Join Table

我怎样才能获得表中每位客户的最后一次销售？

示例（示例数据）：

Customer:
    1, John Smith, jonh@smith.com, ...
    2, Michael Jackson, michael@jackson.com, ...
    3, Bill Clinton, bill@clinton.com, ...

Employee:
   1, Steve Jobs, steve@jobs.com, ...
   2, Bill Gates, bill@gates.com, ...
   3, Mark Zuckerberg, mark@zuckerberg.com, ...

Sales:
   1, 1 (John Smith), 1 (Steve Jobs), '2013-01-02'
   2, 1 (John Smith), 2 (Bill Gates), '2013-01-11'
   3, 1 (John Smith), 3 (Mark Zuckerberg), '2012-12-25'
   4, 2 (Michael Jackson), 2 (Bill Gates), '2012-11-02'
   5, 2 (Michael Jackson), 1 (Steve Jobs), '2013-01-13'
   6, 3 (Bill Clinton), 3 (Mark Zuckerberg), '2013-01-17'
   7, 3 (Bill Clinton), 2 (Bill Gates), '2011-12-20'

我的结果查询必须带来以下结果：

   2, 1 (John Smith), 2 (Bill Gates), '2013-01-11'
   5, 2 (Michael Jackson), 1 (Steve Jobs), '2013-01-13'
   6, 3 (Bill Clinton), 3 (Mark Zuckerberg), '2013-01-17'

Answer 1

由于您已提及使用SQL Server，因此请使用Common Table Expression和Window Function

WITH latestRecords
AS
(
    SELECT  SaleId , CustomerId, EmployeeId , SaleDate,
            DENSE_RANK() OVER (PARTITION BY CustomerId
                                ORDER BY SaleDate DESC) RN
    FROM    Sale
)
SELECT  a.SaleId,
        b.Name AS CustomerName,
        c.Name as EmployeeName,
        a.SaleDate
FROM    latestRecords a
        INNER JOIN Customer b
            ON a.CustomerID = b.CustomerID
        INNER JOIN Employee c
            ON a.EmployeeID = c.EmployeeID
WHERE   a.RN = 1

SQLFiddle Demo

获取连接表中的最后一条记录

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