How Can Recursive Self-Joins in SQL Server Efficiently Navigate Hierarchical Data?

Recursive Self-Joins in SQL Server

In SQL Server, a recursive self-join allows you to navigate hierarchical data structures by joining a table to itself multiple times based on a parent-child relationship.

To perform a recursive self-join to retrieve records related to a specific hierarchy, the following strategy can be employed:

Using a Common Table Expression (CTE)

A CTE, also known as a recursive query, can be used to implement a recursive self-join. The core idea is to define an anchor query that selects records that meet a specific criteria (e.g., no parent) and then use a recursive subquery to append child records to the results:

WITH q AS 
(
    SELECT  *
    FROM    mytable
    WHERE   ParentID IS NULL
    UNION ALL
    SELECT  m.*
    FROM    mytable m
    JOIN    q
    ON      m.parentID = q.PersonID
)
SELECT  *
FROM    q

By adding an ordering condition to the recursive subquery, the tree order can be preserved:

WITH    q AS 
(
    SELECT  m.*, CAST(ROW_NUMBER() OVER (ORDER BY m.PersonId) AS VARCHAR(MAX)) COLLATE Latin1_General_BIN AS bc
    FROM    mytable m
    WHERE   ParentID IS NULL
    UNION ALL
    SELECT  m.*,  q.bc + '.' + CAST(ROW_NUMBER() OVER (PARTITION BY m.ParentID ORDER BY m.PersonID) AS VARCHAR(MAX)) COLLATE Latin1_General_BIN
    FROM    mytable m
    JOIN    q
    ON      m.parentID = q.PersonID
)
SELECT  *
FROM    q
ORDER BY
        bc

Considerations:

• The anchor query should select records that represent the root of the hierarchy you want to traverse.
• The recursive subquery should join the child records with the parent records using the appropriate parent-child relationship column.
• You can add additional columns to the recursive subquery to track level or path information if needed.

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