Etwa so (korrigiert):
SELECT CASE WHEN next.Date IS NULL THEN prev.Rate
WHEN prev.Date IS NULL THEN next.Rate
WHEN next.Date = prev.Date THEN prev.Rate
ELSE ( DATEDIFF(d, prev.Date, @InputDate) * next.Rate
+ DATEDIFF(d, @InputDate, next.Date) * prev.Rate
) / DATEDIFF(d, prev.Date, next.Date)
END AS interpolationRate
FROM
( SELECT TOP 1
Date, Rate
FROM Rates
WHERE Date <= @InputDate
ORDER BY Date DESC
) AS prev
CROSS JOIN
( SELECT TOP 1
Date, Rate
FROM Rates
WHERE Date >= @InputDate
ORDER BY Date ASC
) AS next
