[sql-server] Select SQL results grouped by weeks

I want to select data from following table group by weeks

 Date       Product Name   Sale
+----------+--------------+-----+
 14-05-11     a             2
 14-05-11     b             4 
 17-05-11     c             3
 19-05-11     a             6
 24-05-11     a             6
 29-05-11     a             6    

Let suppose today is 30-05-11

So my result should look like this.

 Product Name         First Week   Second Week  Third Week
+--------------------+------------+------------+-------------+
   a                      12            6           2
   b                       0            0           4 
   c                       0            3           0  

Will some body guide me to how to write SQL query to achieve this behavior!

Base on @increddibelly answer, I applied to my query as below.

I share for whom concerned.

My table structure FamilyData(Id, nodeTime, totalEnergy)

select
   sum(totalEnergy) as TotalEnergy,
   DATEPART ( week, nodeTime ) as weeknr
from FamilyData
group by DATEPART (week, nodeTime)

This should do it for you:

Declare @DatePeriod datetime

Set @DatePeriod = '2011-05-30'

Select  ProductName,
        IsNull([1],0) as 'Week 1',
        IsNull([2],0) as 'Week 2',
        IsNull([3],0) as 'Week 3',
        IsNull([4],0) as 'Week 4',
        IsNull([5], 0) as 'Week 5'

From 
(
Select  ProductName,
        DATEDIFF(week, DATEADD(MONTH, DATEDIFF(MONTH, 0, InputDate), 0), InputDate) +1 as [Weeks],
        Sale as 'Sale'

From dbo.YourTable
-- Only get rows where the date is the same as the DatePeriod
-- i.e DatePeriod is 30th May 2011 then only the weeks of May will be calculated
Where DatePart(Month, InputDate)= DatePart(Month, @DatePeriod)
)p 
Pivot (Sum(Sale) for Weeks in ([1],[2],[3],[4],[5])) as pv

It will calculate the week number relative to the month. So instead of week 20 for the year it will be week 2. The @DatePeriod variable is used to fetch only rows relative to the month (in this example only for the month of May)

Output using my sample data:

Declare @DatePeriod datetime
Set @DatePeriod = '2011-05-30'

Select  ProductName,
        IsNull([1],0) as 'Week 1',
        IsNull([2],0) as 'Week 2',
        IsNull([3],0) as 'Week 3',
        IsNull([4],0) as 'Week 4',
        IsNull([5], 0) as 'Week 5'
From 
(
Select  ProductName,
        DATEDIFF(week, DATEADD(MONTH, DATEDIFF(MONTH, 0, '2011-05-30'), 0), '2011-05-30') +1 as [Weeks],
        Sale as 'Sale'
From dbo.WeekReport

-- Only get rows where the date is the same as the DatePeriod
-- i.e DatePeriod is 30th May 2011 then only the weeks of May will be calculated
Where DatePart(Month, '2011-05-30')= DatePart(Month, @DatePeriod)
)p 
Pivot (Sum(Sale) for Weeks in ([1],[2],[3],[4],[5])) as pv

OUTPUT LOOK LIKE THIS

a   0   0   0   0   20
b   0   0   0   0   4
c   0   0   0   0   3

the provided solutions seem a little complex? this might help:

https://msdn.microsoft.com/en-us/library/ms174420.aspx

select
   mystuff,
   DATEPART ( year, MyDateColumn ) as yearnr,
   DATEPART ( week, MyDateColumn ) as weeknr
from mytable
group by ...etc