Calculate sum of hours from weekly bases?

I need to find the maximum date for that particular client, that will be treated as latest date (or current date). Then, From there I need to go back 52 weeks to 27 weeks and calculate sum(hours). And 26 Weeks to latest date (or current date) and calculate sum(hours).

 ClientID  StoreID    period      hours
    1       10      2010-04-19  8.04
    1       10      2010-04-20  6.24
    1       10      2010-04-21  8.26
    1       20      2010-04-22  7.94
    1       20      2010-04-23  22.43
    1       20      2010-04-24  22.99
    2       5       2010-12-19  130.67
    2       5       2010-12-26  159.26
    2       5       2011-01-02  113.59
    2       5       2011-01-09  12.66
    2       8       2011-01-16  22.34
    2       8       2011-01-23  11.35

Please let me know how to calculate this in SQL 开发者_开发百科to get ouput ? If you have question ask me.

It's simplest to just use two queries for this; the first to get the boundary dates, the second to compute the totals. You could roll it all into one query but I don't think it will make much of a difference.

DECLARE
    @BeginDate datetime,
    @MidDate datetime

SELECT
    @BeginDate = DATEADD(wk, -52, MAX(period)),
    @MidDate = DATEADD(wk, -25, MAX(period))
FROM TableName
WHERE ClientID = @ClientID

SELECT
    SUM(CASE
            WHEN period >= @BeginDate AND period < @MidDate THEN hours
            ELSE 0
        END) AS HoursInFirstHalf,
    SUM(CASE
            WHEN period >= @MidDate THEN hours
            ELSE 0
        END) AS HoursInLastHalf
FROM TableName
WHERE ClientID = @ClientID