Declare @t table (
	Rec1 int,
	Rec2 int
)

Insert Into @t Select 1, 2
Insert Into @t Select 2, 3
Insert Into @t Select 4, 3
Insert Into @t Select 5, 6



Select * From @t


--Select * From @t

;with cte1 As (
	Select * 
	From
		(
		Select * From @t 
		Union All
		Select Rec1 = Rec2, Rec2 = Rec1 From @t
		) A
	Where Rec1 < Rec2
), cte2 As (
	Select *, Grp = Rec1 From cte1
	Union All
	Select B.Rec1, B.Rec2, Grp = A.Rec1 From cte1 A Inner Join cte2 B On A.Rec2 = B.Grp
), cte3 As (
	Select Rec1, Rec2, Grp = MIN(Grp)
	From cte2 
	Group By Rec1, Rec2
)

Select A.*, B.Grp
From @t A
Inner Join cte3 B
On (A.Rec1 = B.Rec1 and A.Rec2 = B.Rec2)
or (A.Rec1 = B.Rec2 and A.Rec2 = B.Rec1)