Sql 规则。最初没有注意到B）。现在添加没有意义，因为您有适合自己的答案。 Date Amount ========= ======= 1/2/2009 1500 2/4/2009 2300 3/15/2009 300 4/1

规则。最初没有注意到B）。现在添加没有意义，因为您有适合自己的答案。

     Date  Amount
========= =======
 1/2/2009    1500
 2/4/2009    2300
3/15/2009     300
4/17/2009    2200
etc.

     Date    Rate
========= =======
 2/1/2009    40.1
 3/1/2009    41.0
 4/1/2009    38.5
 5/1/2009    42.7
etc.

Select 
    t.Date, 
    t.Amount,
    ConvertedAmount=(   
        Select Top 1 
            t.Amount/ex.Rate
        From ExchangeRates ex
        Where t.Date > ex.Date
        Order by ex.Date desc
    )
From Transactions t

WITH IndexedExchangeRates AS (           
            SELECT  Row_Number() OVER (ORDER BY Date) ix,
                    Date,
                    Rate 
            FROM    ExchangeRates 
        ),
        RangedExchangeRates AS (             
            SELECT  CASE WHEN IER.ix=1 THEN CAST('1753-01-01' AS datetime) 
                    ELSE IER.Date 
                    END DateFrom,
                    COALESCE(IER2.Date, GETDATE()) DateTo,
                    IER.Rate 
            FROM    IndexedExchangeRates IER 
            LEFT JOIN IndexedExchangeRates IER2 
            ON IER.ix = IER2.ix-1 
        )
SELECT  T.Date,
        T.Amount,
        RER.Rate,
        T.Amount/RER.Rate ConvertedAmount 
FROM    Transactions T 
LEFT JOIN RangedExchangeRates RER 
ON (T.Date > RER.DateFrom) AND (T.Date <= RER.DateTo)

Select t.Date, t.Amount, 
  ConvertedAmount = t.Amount/coalesce(    
    (Select Top 1 ex.Rate 
        From ExchangeRates ex 
        Where t.Date > ex.Date 
        Order by ex.Date desc )
     ,
     (select top 1 ex.Rate 
        From ExchangeRates  
        Order by ex.Date asc)
    ) 
From Transactions t

SELECT 
    a.tranDate, 
    a.Amount,
    a.Amount/a.Rate as convertedRate
FROM
    (

    SELECT 
        t.date tranDate,
        e.date as rateDate,
        t.Amount,
        e.rate,
        RANK() OVER (Partition BY t.date ORDER BY
                         CASE WHEN DATEDIFF(day,e.date,t.date) < 0 THEN
                                   DATEDIFF(day,e.date,t.date) * -100000
                              ELSE DATEDIFF(day,e.date,t.date)
                         END ) AS diff
    FROM 
        ExchangeRates e
    CROSS JOIN 
        Transactions t
         ) a
WHERE a.diff = 1

 Start Date   End Date    Rate
 ========== ========== =======
 0001-01-01 2009-01-31    40.1
 2009-02-01 2009-02-28    40.1
 2009-03-01 2009-03-31    41.0
 2009-04-01 2009-04-30    38.5
 2009-05-01 9999-12-31    42.7

SELECT T.Date, T.Amount, X.Rate
  FROM Transactions AS T JOIN ExtendedExchangeRates AS X
       ON T.Date BETWEEN X.StartDate AND X.EndDate;

CREATE TABLE ExchangeRate
(
    Date    DATE NOT NULL,
    Rate    DECIMAL(10,5) NOT NULL
);
INSERT INTO ExchangeRate VALUES('2009-02-01', 40.1);
INSERT INTO ExchangeRate VALUES('2009-03-01', 41.0);
INSERT INTO ExchangeRate VALUES('2009-04-01', 38.5);
INSERT INTO ExchangeRate VALUES('2009-05-01', 42.7);

SELECT '0001-01-01' AS StartDate,
       (SELECT MIN(Date) - 1 FROM ExchangeRate) AS EndDate,
       (SELECT Rate FROM ExchangeRate
         WHERE Date = (SELECT MIN(Date) FROM ExchangeRate)) AS Rate
FROM Dual;

0001-01-01  2009-01-31      40.10000

SELECT (SELECT MAX(Date) FROM ExchangeRate) AS StartDate,
       '9999-12-31' AS EndDate,
       (SELECT Rate FROM ExchangeRate
         WHERE Date = (SELECT MAX(Date) FROM ExchangeRate)) AS Rate
FROM Dual;

2009-05-01  9999-12-31      42.70000

SELECT X1.Date     AS StartDate,
       X2.Date - 1 AS EndDate,
       X1.Rate     AS Rate
  FROM ExchangeRate AS X1 JOIN ExchangeRate AS X2
       ON X1.Date < X2.Date
 WHERE NOT EXISTS
       (SELECT *
          FROM ExchangeRate AS X3
         WHERE X3.Date > X1.Date AND X3.Date < X2.Date
        );

2009-02-01  2009-02-28      40.10000
2009-03-01  2009-03-31      41.00000
2009-04-01  2009-04-30      38.50000

2009-02-01  2009-02-28      40.10000
2009-02-01  2009-03-31      40.10000    # Unwanted
2009-02-01  2009-04-30      40.10000    # Unwanted
2009-03-01  2009-03-31      41.00000
2009-03-01  2009-04-30      41.00000    # Unwanted
2009-04-01  2009-04-30      38.50000

SELECT DATE '0001-01-01' AS StartDate,
       (SELECT MIN(Date) - 1 FROM ExchangeRate) AS EndDate,
       (SELECT Rate FROM ExchangeRate
         WHERE Date = (SELECT MIN(Date) FROM ExchangeRate)) AS Rate
FROM Dual
UNION
SELECT X1.Date     AS StartDate,
       X2.Date - 1 AS EndDate,
       X1.Rate     AS Rate
  FROM ExchangeRate AS X1 JOIN ExchangeRate AS X2
       ON X1.Date < X2.Date
 WHERE NOT EXISTS
       (SELECT *
          FROM ExchangeRate AS X3
         WHERE X3.Date > X1.Date AND X3.Date < X2.Date
        )
UNION
SELECT (SELECT MAX(Date) FROM ExchangeRate) AS StartDate,
       DATE '9999-12-31' AS EndDate,
       (SELECT Rate FROM ExchangeRate
         WHERE Date = (SELECT MAX(Date) FROM ExchangeRate)) AS Rate
FROM Dual;

CREATE VIEW ExtendedExchangeRate(StartDate, EndDate, Rate) AS
    SELECT DATE('0001-01-01')  AS StartDate,
           (SELECT MIN(Date) - 1 FROM ExchangeRate) AS EndDate,
           (SELECT Rate FROM ExchangeRate WHERE Date = (SELECT MIN(Date) FROM ExchangeRate)) AS Rate
    FROM Dual
    UNION
    SELECT X1.Date     AS StartDate,
           X2.Date - 1 AS EndDate,
           X1.Rate     AS Rate
      FROM ExchangeRate AS X1 JOIN ExchangeRate AS X2
           ON X1.Date < X2.Date
     WHERE NOT EXISTS
           (SELECT *
              FROM ExchangeRate AS X3
             WHERE X3.Date > X1.Date AND X3.Date < X2.Date
            )
    UNION 
    SELECT (SELECT MAX(Date) FROM ExchangeRate) AS StartDate,
           DATE('9999-12-31') AS EndDate,
           (SELECT Rate FROM ExchangeRate WHERE Date = (SELECT MAX(Date) FROM ExchangeRate)) AS Rate
    FROM Dual;

SELECT      t.Date,
            t.Amount,
            r.Rate
            --//add your multiplication/division here

FROM        "Transactions" t

INNER JOIN  "ExchangeRates" r
        ON  r."ExchangeRateID" = (
                        SELECT TOP 1 x."ExchangeRateID"
                        FROM        "ExchangeRates" x
                        WHERE       x."SourceCurrencyISO" = t."SourceCurrencyISO" --//these are currency-related filters for your tables
                                AND x."TargetCurrencyISO" = t."TargetCurrencyISO" --//,which you should also JOIN on
                                AND x."Date" <= t."Date"
                        ORDER BY    x."Date" DESC)

SELECT
   T.*,
   ExchangeRate = E.Rate
FROM
  dbo.Transactions T
  CROSS APPLY (
    SELECT TOP 1 Rate
    FROM dbo.ExchangeRate E
    WHERE E.RateDate <= T.TranDate
    ORDER BY
      CASE WHEN E.RateDate <= T.TranDate THEN 0 ELSE 1 END,
      E.RateDate DESC
  ) E;

SELECT
   T.*,
   ExchangeRate = Coalesce(E.Rate, E2.Rate)
FROM
  dbo.Transactions T
  OUTER APPLY (
    SELECT TOP 1 Rate
    FROM dbo.ExchangeRate E
    WHERE E.RateDate <= T.TranDate
    ORDER BY E.RateDate DESC
  ) E
  OUTER APPLY (
    SELECT TOP 1 Rate
    FROM dbo.ExchangeRate E2
    WHERE E.Rate IS NULL
    ORDER BY E2.RateDate
  ) E2;