SQL join against date ranges?

Question

Consider two tables:

Transactions, with amounts in a foreign currency:

     Date  Amount
========= =======
 1/2/2009    1500
 2/4/         


        

Answer 1

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

The difference between tran and rate date is calculated, then negative values ( condition b) are multiplied by -10000 so that they can still be ranked but positive values (condition a always take priority. we then select the minimum date difference for each tran date using the rank over clause.