Oracle 11g Release 2

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create table link_tbl as (
  select 'AY' as linkid, 'A' as from_node, 'Y' as to_node from dual union all
  select 'AB', 'A', 'B' from dual union all
  select 'CA', 'C', 'A' from dual union all      
  select 'GE', 'G', 'E' from dual union all
  select 'CB', 'C', 'B' from dual union all
  select 'EF', 'E', 'F' from dual union all
  select 'NM', 'N', 'M' from dual
);
7 rows affected

SELECT * FROM link_tbl;


LINKID
FROM_NODE
TO_NODE


AY
A
Y

AB
A
B

CA
C
A

GE
G
E

CB
C
B

EF
E
F

NM
N
M


WITH
CTE_Nodes
AS
(
    SELECT from_node AS Node
    FROM link_tbl

    UNION

    SELECT to_node AS Node
    FROM link_tbl
)
,CTE_Pairs
AS
(
    SELECT from_node AS Node1, to_node AS Node2
    FROM link_tbl
    WHERE from_node <> to_node

    UNION

    SELECT to_node AS Node1, from_node AS Node2
    FROM link_tbl
    WHERE from_node <> to_node
)
,CTE_Recursive (AnchorNode, Node1, Node2, NodePath, Lvl)
AS
(
    SELECT
        CAST(CTE_Nodes.Node AS varchar(2000)) AS AnchorNode
        , Node1
        , Node2
        , CAST(',' || Node1 || ',' || Node2 || ',' AS varchar(2000)) AS NodePath
        , 1 AS Lvl
    FROM 
        CTE_Pairs
        INNER JOIN CTE_Nodes ON CTE_Nodes.Node = CTE_Pairs.Node1

    UNION ALL

    SELECT 
        CTE_Recursive.AnchorNode
        , CTE_Pairs.Node1
        , CTE_Pairs.Node2
        , CAST(CTE_Recursive.NodePath || CTE_Pairs.Node2 || ',' AS varchar(2000)) AS NodePath
        , CTE_Recursive.Lvl + 1 AS Lvl
    FROM
        CTE_Pairs
        INNER JOIN CTE_Recursive ON CTE_Recursive.Node2 = CTE_Pairs.Node1
    WHERE
        CTE_Recursive.NodePath NOT LIKE CAST('%,' || CTE_Pairs.Node2 || ',%' AS varchar(2000))
)
,CTE_RecursionResult
AS
(
    SELECT AnchorNode, Node1, Node2
    FROM CTE_Recursive
)
,CTE_CleanResult
AS
(
    SELECT AnchorNode, Node1 AS Node
    FROM CTE_RecursionResult

    UNION

    SELECT AnchorNode, Node2 AS Node
    FROM CTE_RecursionResult
)
SELECT
    CTE_Nodes.Node
    ,LISTAGG(CTE_CleanResult.Node, ',') WITHIN GROUP (ORDER BY CTE_CleanResult.Node) AS GroupMembers
    ,DENSE_RANK() OVER (ORDER BY LISTAGG(CTE_CleanResult.Node, ',') WITHIN GROUP (ORDER BY CTE_CleanResult.Node)) AS GroupID
FROM
    CTE_Nodes
    INNER JOIN CTE_CleanResult ON CTE_CleanResult.AnchorNode = CTE_Nodes.Node
GROUP BY
    CTE_Nodes.Node
ORDER BY
    GroupID
    ,CTE_Nodes.Node
;


NODE
GROUPMEMBERS
GROUPID


A
A,B,C,Y
1

B
A,B,C,Y
1

C
A,B,C,Y
1

Y
A,B,C,Y
1

E
E,F,G
2

F
E,F,G
2

G
E,F,G
2

M
M,N
3

N
M,N
3
NODE	GROUPMEMBERS	GROUPID
A	A,B,C,Y	1
B	A,B,C,Y	1
C	A,B,C,Y	1
Y	A,B,C,Y	1
E	E,F,G	2
F	E,F,G	2
G	E,F,G	2
M	M,N	3
N	M,N	3