Best way to interpolate values in SQL

Question

I have a table with rate at certain date :

          Rates

Id  |     Date      |  Rate
----+---------------+-------
 1  |   01/01/2011  |  4.5
 2  |   01/04         


        

Answer 1

The trick with CROSS JOIN here is it wont return any records if either of the table does not have rows (1 * 0 = 0) and the query may break. Better way to do is use FULL OUTER JOIN with inequality condition (to avoid getting more than one row)

( SELECT TOP 1 
        Date, Rate 
    FROM Rates
    WHERE Date <= @InputDate
    ORDER BY Date DESC
  ) AS prev
  FULL OUTER JOIN
  ( SELECT TOP 1 
        Date, Rate 
    FROM Rates
    WHERE Date >= @InputDate
    ORDER BY Date ASC
  ) AS next
ON (prev.Date <> next.Date) [or Rate depending on what is unique]