Declare @myTable table (Root_Id int, Parent_Node_Id int, Child_Node_Id int, Path varchar(100))
Insert Into @myTable Values (159,545,547,Null)
Insert Into @myTable Values (159,545,548,Null)
Insert Into @myTable Values (159,546,549,Null)
Insert Into @myTable Values (159,547,550,Null)
Insert Into @myTable Values (159,547,551,Null)
Insert Into @myTable Values (159,547,552,Null)
Insert Into @myTable Values (159,549,553,Null)
Insert Into @myTable Values (159,546,548,Null)
Insert Into @myTable Values (159,549,552,Null)


Select * From @myTable

Declare @tree table (pNodeId int, cNodeId int, processed bit default(0))
Insert Into @Tree
Select pNodeId = parent_Node_Id, cNodeId = child_node_Id, processed = 0 From @myTable

/*****************************************
You didn't have a starting point so I had to add it.
********************************************/
Insert Into @tree
Select pNodeId = 159, cNodeId = 545, Processed = 0
Union All
Select pNodeId = 159, cNodeId = 546, Processed = 0


Declare @pad nvarchar(100),
	@lastCnt int
Set @Pad = '0000'

Declare @paths table (path nvarchar(1000), pNodeId int, cNodeId int)

Insert Into @paths
Select 
	path=right(@pad + convert(nvarchar,pNodeId),len(@pad))+';' + right(@pad + convert(nvarchar,cNodeId),len(@pad))+';',
	pNodeId,
	cNodeId
From @Tree
where pNodeId=159

Update A
Set Processed = 1
From @Tree as A
Inner Join @paths as B
On A.pNodeId = B.pNodeId
and A.cNodeId = B.cNodeId

While exists(Select * From @tree Where Processed = 0)
Begin
	Insert Into @paths
	Select 
		path=path + case when B.cNodeId is not null then right(@pad + convert(nvarchar,B.cNodeId),len(@pad))+';' else '' end,
		B.pNodeId,
		B.cNodeId
	From @Paths as A
	Left Join @Tree as B
	On A.cNodeId = B.pNodeId
	Where B.Processed = 0

	Update A
	Set Processed = 1
	From @Tree as A
	Inner Join @paths as B
	On A.pNodeId = B.pNodeId
	and A.cNodeId = B.cNodeId
	Where A.Processed = 0
End

Update A
Set Path = B.path 
From @myTable as A
inner Join @paths as B
On A.parent_Node_Id = B.pNodeId
and A.child_Node_Id = B.cNodeId

Select * From @myTable