Complicated AVG within date range - tsql

Question

I've got a table with a tracking of a plant's equipment installation.
Here is a sample:
ID Name Date Percentage
1 GT-001 2011-01-08 30
2 GT-002 2011-01-11 40
3 GT-003 2011-02-02 30
4 GT-001 2011-02-03 50
5 GT-003 2011-02-15 50
6 GT-004 2011-02-15 30
7 GT-002 2011-02-15 60
8 GT-001 2011-02-20 60
9 GT-003 2011-03-01 60
10 GT-004 2011-03-05 50
11 GT-001 2011-03-10 70
12 GT-004 2011-03-15 60
And the corresponding script:
CREATE TABLE [dbo].[SampleTable](
[ID] [int] NOT NULL,
[Name] [nvarchar](50) NULL,
[Date] [date] NULL,
[Percentage] [int] NULL) ON [PRIMARY]
GO
--Populate the table with values
INSERT INTO [dbo].[SampleTable] VALUES
('1', 'GT-001', '2011-01-08', '30'),
('2', 'GT-002', '2011-01-11', '40'),
('3', 'GT-003', '2011-02-02', '30'),
('4', 'GT-001', '2011-02-03', '50'),
('5', 'GT-003', '2011-02-15', '50'),
('6', 'GT-004', '2011-02-15', '30'),
('7', 'GT-002', '2011-02-15', '60'),
('8', 'GT-001', '2011-02-20', '60'),
('9', 'GT-003', '2011-03-01', '60'),
('10', 'GT-004', '2011-03-05', '50'),
('11', 'GT-001', '2011-03-10', '70'),
('12', 'GT-004', '2011-03-15', '60');
GO
What i need is to create a chart with Date on the X and Average Percentage on the Y. Average Percentage is an average percentage of all equipment by that particular date starting from the beggining of the installation process (MIN(Fields!Date.Value, "EquipmentDataset"))
Having no luck in implementing this using SSRS only, i decided to create a more complicated dataset for it using T-SQL.
I guess that it is nessesary to add a calculated column named 'AveragePercentage' that should store an average percentage on that date, calculating only the most latest equipment percentage values in a range between the beggining of the installation process (MIN(Date)) and the current row's date. Smells like a recursion, but i'm newbie to T-SQL....))
Here is the desired output
ID Name Date Percentage Average
1 GT-001 2011-01-08 30 30
2 GT-002 2011-01-11 40 35
3 GT-003 2011-02-02 30 33
4 GT-001 2011-02-03 50 40
5 GT-003 2011-02-15 50 48
6 GT-004 2011-02-15 30 48
7 GT-002 2011-02-15 60 48
8 GT-001 2011-02-20 60 50
9 GT-003 2011-03-01 60 53
10 GT-004 2011-03-05 50 58
11 GT-001 2011-03-10 70 60
12 GT-004 2011-03-15 60 63
What do you think?
I'll be very appreciated for any help.

Answer 1

You could use cross apply with row_number to find the latest value for each machine. An additional subquery is required because you cannot use row_number in the where clause directly. Here's the query:
select t1.id
, t1.Name
, t1.Date
, t1.Percentage
, avg(1.0*last_per_machine.percentage)
from SampleTable t1
outer apply
(
select *
from (
select row_number() over (partition by Name order by id desc)
as rn
, *
from SampleTable t2
where t2.date <= t1.date
) as numbered
where rn = 1
) as last_per_machine
group by
t1.id
, t1.Name
, t1.Date
, t1.Percentage
Working example on SE Data.