难以离开 T-SQL 连接？

Question

我有一张 ExchangeRates 表，其中包含一个 countryid 和一个 exchangerateate 的东西：

ExchangeRateID   Country   ToUSD      ExchangeRateDate
1                  Euro     .7400     2/14/2011
2                  JAP      80.1900   2/14/2011
3                  Euro     .7700      7/20/2011

请注意，同一国家/地区可能会根据日期使用不同的汇率...例如，上面的欧元是 2011 年 2 月 14 日的 0.7400，现在是 2011 年 7 月 20 日的 0.7700。

我有另一个订单项表，用于根据国家/地区列出项目。在此表中，每个订单项都有一个与之关联的日期。行项目日期应使用基于汇率的相应日期和国家。因此，如果我在 2011 年 2 月 16 日有一个带有国家欧元的订单项，则使用上述数据，它应该使用 2011 年 2 月 14 日的欧元值，而不是 2011 年 7 月 20 日的值，因为日期（条件 er. ExchangeRateDate

SELECT     
    er.ExchangeRateID, 
    er.CountryID AS Expr1, 
    er.ExchangeRateDate, 
    er.ToUSD, 
    erli.ExpenseReportLineItemID, 
    erli.ExpenseReportID, 
    erli.LineItemDate
FROM         
    dbo.ExpenseReportLineItem AS erli 
LEFT JOIN
    dbo.ExchangeRate AS er 
ON er.CountryID = erli.CountryID 
AND DATEADD(d, DATEDIFF(d, 0, er.ExchangeRateDate), 0) <= DATEADD(d, DATEDIFF(d, 0, 
                      erli.LineItemDate), 0)
WHERE     (erli.ExpenseReportID = 196)

这个左连接的问题...是因为日期是

LineItem 表有多条记录，每条记录都可以有自己的 CountryID：

Item            Country      ParentID    LineItemDate
Line Item 1      Euro           1           2/14/2011
Line Item 2      US             1           2/14/2011
Line Item3       Euro           1           2/15/2011

所以 ParentID (ExpenseReportID) = 1 有 3 条记录。然后我获取这些记录并加入 ExchangeRate 表，其中我的行项目表中的 Country = 汇率表的国家/地区（这部分很简单）但是我要做的第二个条件是：

  AND DATEADD(d, DATEDIFF(d, 0, er.ExchangeRateDate), 0) <= DATEADD(d, DATEDIFF(d, 0, 
                          erli.LineItemDate), 0)

但这就是问题所在，因为这将返回我的汇率表中的多行，因为欧元被列出了两次。

Answer 1

我可能在这里遗漏了一些东西，但据我了解，解决您的问题的“愚蠢”解决方案是使用 A ROW_NUMBER 函数和外部过滤器与您现有的“返回太多条目”查询（这也可以通过CTE，但对于像这样的简单情况，我更喜欢派生表语法）：

SELECT *
FROM (
    SELECT     
        er.ExchangeRateID, 
        er.CountryID AS Expr1, 
        er.ExchangeRateDate, 
        er.ToUSD, 
        erli.ExpenseReportLineItemID, 
        erli.ExpenseReportID, 
        erli.LineItemDate,
        ROW_NUMBER() OVER (PARTITION BY ExpenseReportID, ExpenseReportLineItemID ORDER BY ExchangeRateDate DESC) AS ExchangeRateOrderID
    FROM dbo.ExpenseReportLineItem AS erli 
    LEFT JOIN dbo.ExchangeRate AS er 
        ON er.CountryID = erli.CountryID 
            AND DATEADD(d, DATEDIFF(d, 0, er.ExchangeRateDate), 0) 
                <= DATEADD(d, DATEDIFF(d, 0, erli.LineItemDate), 0)
    WHERE (erli.ExpenseReportID = 196)
        --For reasonable performance, it would be VERY nice to put a filter
        -- on how far back the exchange rates can go here:
        --AND er.ExchangeRateDate > DateAdd(Day, -7, GetDate())
) As FullData
WHERE ExchangeRateOrderID = 1

如果我误解了，请见谅，否则希望对您有所帮助！

Answer 2

如果您可以在 ExchangeRates 表中添加一个名为（类似）的附加列，您的生活会轻松很多

ExchangeRateToDate

一个单独的进程可以在添加新条目时更新之前的条目。

然后，您可以只查询 LineItemDate >= ExhangeRateDate 和

（将最后一个，可能是 null ExchangeRateToDate 视为特殊情况）。

Answer 3

我将创建一个内存表，并使用 ExchangeRateDates From & To 创建一个 ExchangeRate 表。
之后剩下要做的就是在查询中加入此 CTE 而不是 ExchangeRate 表，并添加日期为 betweenthe date from/to 的条件。

SQL 语句

;WITH er AS (
    SELECT  rn = ROW_NUMBER() OVER (PARTITION BY er1.ExchangeRateID ORDER BY er2.ExchangeRateDate DESC)
            , er1.ExchangeRateID
            , er1.Country
            , ExchangeRateDateFrom = ISNULL(DATEADD(d, 1, er2.ExchangeRateDate), 0)
            , ExchangeRateDateTo = er1.ExchangeRateDate
            , er1.ToUSD
    FROM    @ExchangeRate er1
            LEFT OUTER JOIN @ExchangeRate er2
                ON  er1.Country = er2.Country
                    AND er1.ExchangeRateDate >= er2.ExchangeRateDate
                    AND er1.ExchangeRateID > er2.ExchangeRateID     
)
SELECT  er.ExchangeRateID, 
        er.CountryID AS Expr1, 
        er.ExchangeRateDateTo, 
        er.ToUSD, 
        erli.ExpenseReportLineItemID, 
        erli.ExpenseReportID, 
        erli.LineItemDate
FROM    dbo.ExpenseReportLineItem AS erli 
        LEFT JOIN er ON er.CountryID = erli.CountryID 
                        AND DATEADD(d, DATEDIFF(d, 0, er.ExchangeRateDateTo), 0) <= DATEADD(d, DATEDIFF(d, 0, erli.LineItemDate), 0)
                        AND DATEADD(d, DATEDIFF(d, 0, er.ExchangeRateDateFrom), 0) >= DATEADD(d, DATEDIFF(d, 0, erli.LineItemDate), 0)
WHERE   (erli.ExpenseReportID = 196)
        and er.rn = 1

测试脚本

DECLARE @ExchangeRate TABLE (
    ExchangeRateID INTEGER
    , Country VARCHAR(32)
    , ToUSD FLOAT
    , ExchangeRateDate DATETIME
)   

INSERT INTO @ExchangeRate 
VALUES  (1, 'Euro', 0.7400, '02/14/2011')
        , (2, 'JAP', 80.1900, '02/14/2011')
        , (3, 'Euro', 0.7700, '07/20/2011')     
        , (4, 'Euro', 0.7800, '07/25/2011')     

;WITH er AS (
    SELECT  rn = ROW_NUMBER() OVER (PARTITION BY er1.ExchangeRateID ORDER BY er2.ExchangeRateDate DESC)
            , er1.ExchangeRateID
            , er1.Country
            , ExchangeRateDateFrom = ISNULL(DATEADD(d, 1, er2.ExchangeRateDate), 0)
            , ExchangeRateDateTo = er1.ExchangeRateDate
            , ToUSD = er1.ToUSD
    FROM    @ExchangeRate er1
            LEFT OUTER JOIN @ExchangeRate er2
                ON  er1.Country = er2.Country
                    AND er1.ExchangeRateDate >= er2.ExchangeRateDate
                    AND er1.ExchangeRateID > er2.ExchangeRateID     
)
SELECT  *
FROM    er
WHERE   rn = 1

Answer 4

也许您可以尝试使用table expression 来获得您的TOP 1，然后加入表表达式。那有意义吗？希望这可以帮助。

Answer 5

这可以通过使用一个或多个 CTE 来解决。这个较早的 SO 问题应该具有所需的构建块： How can you use SQL to return values for a specified date or closest date < specified date?

请注意，您必须将其修改为您自己的架构，并过滤掉更接近但在未来的结果。 我希望这会有所帮助，但如果还不够，我相信我可以发布更详细的答案。

Answer 6

如果我没有误解你想要做什么，你可以使用外部申请来获得最新的汇率。

select *
from ExpenseReportLineItem erli
  outer apply (select top 1 *
               from ExchangeRates as er1
               where er1.Country = erli.Country and
                     er1.ExchangeRateDate <= erli.LineItemDate 
               order by er1.ExchangeRateDate desc) as er

Answer 7

您可以将其用作相关子查询，它会为您提供一个表格，其中包含给定日期的最新交换值（在评论中指出）：

SELECT * 
FROM er
    INNER JOIN
    (
        SELECT CountryID, MAX(ExchangeRateDate) AS ExchangeRateDate
        FROM er
        WHERE ExchangeRateDate <= '9/1/2011' 
            -- the above is the date you will need to correlate with the main query...
        GROUP BY Country
    ) iq
    ON iq.Country = er.Country AND er.ExchangeRateDate = iq.ExchangeRateDate

所以完整的查询应该是这样的：

SELECT     
    iq2.ExchangeRateID, 
    iq2.CountryID AS Expr1, 
    iq2.ExchangeRateDate, 
    iq2.ToUSD, 
    erli.ExpenseReportLineItemID, 
    erli.ExpenseReportID, 
    erli.LineItemDate
FROM dbo.ExpenseReportLineItem AS erli 
    LEFT JOIN
    (
        SELECT * 
        FROM ExchangeRate er
            INNER JOIN
            (
                SELECT CountryID, MAX(ExchangeRateDate) AS ExchangeRateDate
                FROM ExchangeRate er
                WHERE ExchangeRateDate <= erli.LineItemDate 
                -- the above is where the correlation occurs...
                GROUP BY Country
            ) iq
            ON iq.Country = er.Country AND er.ExchangeRateDate = iq.ExchangeRateDate
    ) iq2
    ON er.CountryID = erli.CountryID 
        AND DATEADD(d, DATEDIFF(d, 0, iq2.ExchangeRateDate), 0) <= DATEADD(d, DATEDIFF(d, 0, erli.LineItemDate), 0)
    WHERE     (erli.ExpenseReportID = 196)