How Can I Perform Recursive Self-Joins in SQL Server to Explore Hierarchical Data?

Mastering Recursive Self-Joins in SQL Server for Hierarchical Data

Recursive self-joins are essential for navigating hierarchical data structures within SQL Server tables. This technique is particularly useful when dealing with data organized in parent-child relationships, such as organizational charts or family trees.

A straightforward approach to recursive self-joins uses a common table expression (CTE):

<code class="language-sql">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;</code>

This query defines a recursive CTE, q. The initial SELECT statement identifies the root nodes (those with ParentID as NULL). The UNION ALL combines this with subsequent SELECT statements that recursively join the CTE with the mytable to include all descendants. The final SELECT statement retrieves the complete hierarchical dataset.

Maintaining Hierarchical Order

To preserve the original hierarchical structure and order, a modified query is needed:

<code class="language-sql">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;</code>

This enhanced query adds a bc column using ROW_NUMBER() to assign unique identifiers, maintaining the order within each level of the hierarchy. The ORDER BY bc clause ensures the final result reflects the original tree structure. Adjusting the ORDER BY clauses within the ROW_NUMBER() function allows for customization of sibling node ordering.

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