Recursion in User Defined Functions (UDFs) can be a powerful tool to solve problems that have a recursive nature, such as hierarchical data or mathematical computations. In SQL Server, you can implement recursive logic within UDFs, particularly using Table-Valued Functions (TVFs).

Example: Calculating Factorial Using a Recursive Function

Here's an example of a scalar UDF to calculate the factorial of a number using recursion:

CREATE FUNCTION dbo.Factorial(@n INT)
RETURNS INT
AS
BEGIN
    -- Base case
    IF @n <= 1
        RETURN 1;

    -- Recursive case
    RETURN @n * dbo.Factorial(@n - 1);
END;

Example: Recursive Common Table Expression (CTE) for Hierarchical Data

For more complex recursion, such as traversing hierarchical data, you might prefer using a Recursive Common Table Expression (CTE) within a function. Here' an example to find all employees and their respective managers:

Assume we have a table Employees:

CREATE TABLE Employees (
    EmployeeID INT PRIMARY KEY,
    EmployeeName VARCHAR(50),
    ManagerID INT
);

INSERT INTO Employees (EmployeeID, EmployeeName, ManagerID) VALUES
(1, 'Alice', NULL),
(2, 'Bob', 1),
(3, 'Charlie', 1),
(4, 'David', 2),
(5, 'Eve', 2);

We can create a UDF using a CTE to get the hierarchical structure:

CREATE FUNCTION dbo.GetEmployeeHierarchy(@EmployeeID INT)
RETURNS @Hierarchy TABLE (
    EmployeeID INT,
    EmployeeName VARCHAR(50),
    ManagerID INT,
    Level INT
)
AS
BEGIN
    WITH EmployeeCTE AS (
        SELECT EmployeeID, EmployeeName, ManagerID, 0 AS Level
        FROM Employees
        WHERE EmployeeID = @EmployeeID
        UNION ALL
        SELECT e.EmployeeID, e.EmployeeName, e.ManagerID, Level + 1
        FROM Employees e
        INNER JOIN EmployeeCTE ec ON e.ManagerID = ec.EmployeeID
    )
    INSERT INTO @Hierarchy
    SELECT EmployeeID, EmployeeName, ManagerID, Level
    FROM EmployeeCTE;

    RETURN;
END;

Using the Function

You can call this function to get the hierarchy for a specific employee:

SELECT * FROM dbo.GetEmployeeHierarchy(1);

Considerations

• Performance: Recursive functions can be expensive in terms of performance, especially if not managed carefully. Be cautious with deep recursion.

• Max Recursion: SQL Server has a default maximum recursion depth of 100 for CTEs. You can change this limit using the OPTION (MAXRECURSION n) query hint, where n is the new limit.

• Error Handling: Ensure your function handles base cases and edge cases properly to avoid infinite loops and stack overflow errors.

Using recursion in UDFs can be highly effective for certain types of problems. If you have specific use cases or need further examples, feel free to let me know!