What is the simplest way of doing a recursive self-join in SQL Server?

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Recursive Self-Join in SQL Server

Handling hierarchical data, like organizational structures or category trees, often requires querying a table that references itself. The simplest way to perform a recursive self-join in SQL Server is by using a Common Table Expression (CTE). This approach allows you to traverse and query hierarchical relationships efficiently.

Example Scenario

Imagine you have an Employees table where each employee has a ManagerID that references another employee in the same table. You want to list all employees along with their hierarchical levels within the organization.

What is a Recursive Self-Join

A recursive self-join is a technique where a table joins to itself to navigate hierarchical relationships. Using a recursive CTE, you can iteratively build the hierarchy by repeatedly joining the table to itself until all levels are covered.

Steps to Perform a Recursive Self-Join

1. Define the Employees Table

First, create and populate the Employees table with sample data.

CREATE TABLE Employees (
    EmployeeID INT PRIMARY KEY,
    EmployeeName VARCHAR(100),
    ManagerID INT NULL,
    FOREIGN KEY (ManagerID) REFERENCES Employees(EmployeeID)
);

INSERT INTO Employees (EmployeeID, EmployeeName, ManagerID) VALUES
(1, 'CEO', NULL),
(2, 'CTO', 1),
(3, 'CFO', 1),
(4, 'Dev Manager', 2),
(5, 'Developer', 4),
(6, 'Accountant', 3);

2. Use a Recursive CTE

A Common Table Expression (CTE) can be used to perform the recursive self-join. Here's how to set it up:

WITH EmployeeHierarchy AS (
    -- Anchor member: select the top-level employee(s)
    SELECT
        EmployeeID,
        EmployeeName,
        ManagerID,
        0 AS Level
    FROM
        Employees
    WHERE
        ManagerID IS NULL

    UNION ALL

    -- Recursive member: select employees reporting to the current level
    SELECT
        e.EmployeeID,
        e.EmployeeName,
        e.ManagerID,
        eh.Level + 1
    FROM
        Employees e
    INNER JOIN
        EmployeeHierarchy eh ON e.ManagerID = eh.EmployeeID
)
SELECT
    EmployeeID,
    EmployeeName,
    ManagerID,
    Level
FROM
    EmployeeHierarchy
ORDER BY
    Level, EmployeeName;

Explanation

Anchor Member: Selects the top-level employees (those without a manager).
Recursive Member: Joins the Employees table with the CTE (EmployeeHierarchy) to find employees managed by those already in the hierarchy.
SELECT Statement: Retrieves the hierarchical list with levels indicating the depth in the hierarchy.

Output

EmployeeID	EmployeeName	ManagerID	Level
1	CEO	NULL	0
2	CTO	1	1
3	CFO	1	1
4	Dev Manager	2	2
6	Accountant	3	2
5	Developer	4	3

Benefits of Using Recursive Self-Joins

Simplicity: Recursive CTEs make it easy to traverse hierarchical data without complex joins.
Performance: Efficiently handles multiple levels of hierarchy with minimal code.
Readability: The query structure is clear and easy to understand, facilitating maintenance.

Considerations

Recursion Limit: SQL Server has a default recursion limit of 100. You can adjust this using the OPTION (MAXRECURSION n) hint if your hierarchy is deeper.
Performance: For very large or deep hierarchies, performance might be impacted. Ensure proper indexing and optimize your queries as needed.