SQL Server – Calculate Time Difference Between Datetime and Current Date & Time

Instead of your hack with replace, why not just cast to the right data type? This doesn't need to be nvarchar and the only reason it contains time is because you included too many characters. You can of course apply your replace here as well (or you could do that at the client).

SELECT CONVERT(CHAR(11),GETDATE(),13)
UNION ALL
SELECT CONVERT(CHAR(11), DATEADD(DAY, 10, GETDATE()), 13);

Results:

27 Aug 2012
06 Sep 2012

I used your SQL Fiddle and posted a working solution here.

WITH Calc AS
(
    SELECT
        S.Pair,
        S.Date,
        S.Time,
        S.Bid,
        ABS(S.Bid - T.Bid) AS Diff
    FROM dbo.forexData AS S
    CROSS APPLY
    (
        SELECT TOP (1)
            T.Bid
        FROM dbo.forexData AS T
        WHERE
            T.Id > S.Id
            AND t.Pair = s.Pair
        ORDER BY
            T.Bid DESC
    ) AS T
)
SELECT
    Pair,
    AVG(DIFF)
FROM Calc
GROUP BY
    Pair;

To summarize:

• I fixed a hole in your existing logic - currently your CROSS APPLY doesn't link to the same pair value so if you have multiple pairs then your DIFF could be incorrect.
• I put the fixed logic into a CTE
• I queried the CTE with a GROUP BY on pair
• I added an ORDER BY specification to determine the TOP ordering

You can add a date filter in a WHERE to the outer SELECT if you desire.