SQL join against date ranges?

You could first do a self-join on the exchange rates which are ordered by date so that you have the start and the end date of each exchange rate, without any overlap or gap in the dates (maybe add that as view to your database – in my case I’m just using a common table expression).

Now joining those “prepared” rates with the transactions is simple and efficient.

Something like:

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)

Notes:

• You could replace GETDATE() with a date in the far future, I’m assuming here that no rates for the future are known.

• Rule (B) is implemented by setting the date of the first known exchange rate to the minimal date supported by the SQL Server datetime, which should (by definition if it is the type you’re using for the Date column) be the smallest value possible.