How to group hierarchical relationships together in SQL Server

Question

I have a column name Parent and Child in table Example and Below is the Table Data

|     Parent          |         


        

Answer 1

Here is a cut-down version of the query from a more generic question How to find all connected subgraphs of an undirected graph

The main idea is to treat (Parent,Child) pairs as edges in a graph and traverse all connected edges starting from a given node.

Since the graph is undirectional we build a list of pairs in both directions in CTE_Pairs at first.

CTE_Recursive follows the edges of a graph and stops when it detects a loop. It builds a path of visited nodes as a string in IDPath and stops the recursion if the new node is in the path (has been visited before).

Final CTE_CleanResult puts all found nodes in one simple list.

CREATE PROCEDURE GetAncestors(@thingID varchar(8000))
AS
BEGIN
    SET NOCOUNT ON;

    WITH
    CTE_Pairs
    AS
    (
        SELECT
            CAST(Parent AS varchar(8000)) AS ID1
            ,CAST(Child AS varchar(8000)) AS ID2
        FROM Example
        WHERE Parent <> Child

        UNION

        SELECT
            CAST(Child AS varchar(8000)) AS ID1
            ,CAST(Parent AS varchar(8000)) AS ID2
        FROM Example
        WHERE Parent <> Child
    )
    ,CTE_Recursive
    AS
    (
        SELECT
            ID1 AS AnchorID
            ,ID1
            ,ID2
            ,CAST(',' + ID1 + ',' + ID2 + ',' AS varchar(8000)) AS IDPath
            ,1 AS Lvl
        FROM
            CTE_Pairs
        WHERE ID1 = @thingID

        UNION ALL

        SELECT
            CTE_Recursive.AnchorID
            ,CTE_Pairs.ID1
            ,CTE_Pairs.ID2
            ,CAST(CTE_Recursive.IDPath + CTE_Pairs.ID2 + ',' AS varchar(8000)) AS IDPath
            ,CTE_Recursive.Lvl + 1 AS Lvl
        FROM
            CTE_Pairs
            INNER JOIN CTE_Recursive ON CTE_Recursive.ID2 = CTE_Pairs.ID1
        WHERE
            CTE_Recursive.IDPath NOT LIKE '%,' + CTE_Pairs.ID2 + ',%'
    )
    ,CTE_RecursionResult
    AS
    (
        SELECT AnchorID, ID1, ID2
        FROM CTE_Recursive
    )
    ,CTE_CleanResult
    AS
    (
        SELECT AnchorID, ID1 AS ID
        FROM CTE_RecursionResult

        UNION

        SELECT AnchorID, ID2 AS ID
        FROM CTE_RecursionResult
    )
    SELECT ID
    FROM CTE_CleanResult
    ORDER BY ID
    OPTION(MAXRECURSION 0);

END;