Изберете SQL резултати, групирани по седмици

Мисля, че това трябва да го направи...

Select 
ProductName,
WeekNumber,
sum(sale)
from
(
    SELECT 
    ProductName,
    DATEDIFF(week, '2011-05-30', date) AS WeekNumber,
    sale
    FROM table
)
GROUP BY
ProductName,
WeekNumber