How to Retrieve a Category and Its Ancestors Using Recursive Self-Join in SQL Server?

Using Recursive Self-Joins in SQL Server to Retrieve Categories and Their Ancestors

This article demonstrates how to retrieve a category and its ancestor categories from a hierarchical table in SQL Server using a recursive self-join. We'll use a Common Table Expression (CTE) for this task.

Scenario:

Consider a Categories table with Id, Name, and ParentId columns representing a hierarchical category structure. The goal is to write a query that returns a specific category (e.g., "Business Laptops") along with all its parent categories in a single, comma-separated string.

Recursive CTE Solution:

The solution employs a recursive CTE to traverse the hierarchical structure. Here's the query:

WITH CategoryHierarchy AS (
    SELECT id, name, CAST(name AS VARCHAR(MAX)) AS path, parent_id
    FROM Categories
    WHERE parent_id IS NULL  -- Start with root categories
    UNION ALL
    SELECT c.id, c.name, CAST(ch.path + ',' + c.name AS VARCHAR(MAX)), c.parent_id
    FROM Categories c
    INNER JOIN CategoryHierarchy ch ON c.parent_id = ch.id
)

This CTE, CategoryHierarchy, recursively joins the Categories table to itself. The initial SELECT statement selects root categories (those with parent_id as NULL). The UNION ALL combines this with subsequent recursive selections, building the path string by concatenating parent and child category names.

Query to Retrieve Specific Category and Ancestors:

To retrieve the "Business Laptops" category and its ancestors:

SELECT id, name, path
FROM CategoryHierarchy
WHERE name = 'Business Laptops';

Example Table and Data:

Let's create a sample Categories table and insert some data:

CREATE TABLE Categories (
    Id INT PRIMARY KEY,
    Name VARCHAR(100),
    ParentId INT REFERENCES Categories(Id)
);

INSERT INTO Categories (Id, Name, ParentId)
VALUES
(1, 'Electronics', NULL),
(2, 'Laptops', 1),
(3, 'Desktops', 1),
(4, 'Business Laptops', 2),
(5, 'Gaming Laptops', 2);

Expected Result:

For the query targeting 'Business Laptops', the expected output would be:

<code>id      name                path
4       Business Laptops     Electronics,Laptops,Business Laptops</code>

This approach effectively retrieves a category and its complete lineage using a recursive CTE, providing a clear and efficient solution for navigating hierarchical data in SQL Server. Remember to adjust the VARCHAR(MAX) length if your category names might exceed this limit.

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