How do I select Top 1 values of one column when there is two columns in the select clause

USE [MASTER]
GO
/****** Object:  Table [dbo].[testing_new]    Script Date: 03/13/2014 19:39:39 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[testing_new](
	[id] [int] NULL,
	[pid] [int] NULL
) ON [PRIMARY]
GO
INSERT [dbo].[testing_new] ([id], [pid]) VALUES (1, 100)
INSERT [dbo].[testing_new] ([id], [pid]) VALUES (2, 101)
INSERT [dbo].[testing_new] ([id], [pid]) VALUES (3, 102)
INSERT [dbo].[testing_new] ([id], [pid]) VALUES (1, 103)
INSERT [dbo].[testing_new] ([id], [pid]) VALUES (1, 104)
INSERT [dbo].[testing_new] ([id], [pid]) VALUES (2, 105)
INSERT [dbo].[testing_new] ([id], [pid]) VALUES (4, 106)



;with a as
(
SELECT [ID],[PID], 
rn = row_number() over (partition by [ID] order by ID)
FROM testing_new
)
select 
[ID],[PID] from a
where rn = 1 

SELECT ID,PID
FROM
(SELECT  ID,PID, ROW_NUMBER() OVER (PARTITION BY ID,PID ORDER BY ID) AS RowNUM FROM Mytable) AS MT
WHERE MT.RowNUM =1