Calculate sum of hours from weekly bases?
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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? If you have question ask me.