Recursion is a powerful concept in programming that allows a function or procedure to call itself. In SQL Server, recursion can be used to solve complex problems and simplify development. In this article, we will explore recursion in SQL Server and its implementations, complete with examples.

What is Recursion?

Recursion is an algorithmic approach where a function or procedure calls itself to solve a problem. It involves breaking down a problem into smaller subproblems and solving them recursively until a base case is reached. The base case is the condition that stops the recursive calls and returns the final result.

One classic example of recursion is the factorial algorithm. The factorial of a positive integer n is the product of all positive integers less than or equal to n. We can write a recursive algorithm to calculate the factorial:

function fact(n)
{
   if (n > 1)
      return (n * fact(n - 1)); -- recursive call
   else
      return 1;                  -- base case
}

Using recursion, we can calculate the factorial of a number in a concise and elegant way.

Recursion in SQL Server

Starting with SQL Server 2005, developers have had recursion available as a T-SQL language feature. There are three main implementations of recursion in SQL Server:

1. Recursive Stored Procedures
2. Recursive Functions
3. Recursive Common Table Expressions (CTEs)

Recursive Stored Procedures

A recursive stored procedure is a stored procedure that calls itself a finite number of times. It can be used to solve complex problems that require iterative calculations. Let’s take the factorial example and implement it as a recursive stored procedure:

CREATE PROCEDURE SP_factorial
    @param INT,
    @fact BIGINT OUTPUT
AS
BEGIN
    IF (@param >= 0 AND @param <= 20)
    BEGIN
        IF (@param > 1)
            EXEC SP_factorial @param - 1, @fact OUTPUT;
        ELSE
            SET @fact = 1;
        
        SET @fact = @fact * @param;
    END
    ELSE
        SET @fact = NULL;
END;

This stored procedure calculates the factorial for an integer between 0 and 20. It uses a recursive call to calculate the factorial step by step until the base case is reached.

Recursive Functions

Similar to recursive stored procedures, recursive functions in SQL Server can call themselves to solve a problem. Let’s implement the factorial example as a recursive function:

CREATE FUNCTION FN_factorial (@param INT)
RETURNS BIGINT
AS
BEGIN
    DECLARE @fact BIGINT;
    
    IF (@param >= 0 AND @param <= 20)
    BEGIN
        IF (@param > 1)
            SET @fact = @param * dbo.FN_factorial(@param - 1);
        ELSE
            SET @fact = 1;
        
        RETURN @fact;
    END
    ELSE
        RETURN NULL;
END;

This function calculates the factorial for an integer between 0 and 20. It uses a recursive call to calculate the factorial step by step until the base case is reached.

Recursive Common Table Expressions (CTEs)

A recursive Common Table Expression (CTE) is a temporary result set that can be used within a query. It allows for recursive queries by referencing itself in the query definition. Let’s implement the factorial example as a recursive CTE:

CREATE FUNCTION FN_CTE_factorial (@param INT)
RETURNS TABLE
AS
RETURN
(
    WITH RecursiveCTE (param_val, fact) AS
    (
        SELECT 0, 1 -- Base case
        UNION ALL
        SELECT param_val + 1, (param_val + 1) * fact
        FROM RecursiveCTE
        WHERE param_val < @param
    )
    SELECT param_val, fact
    FROM RecursiveCTE
);

This function calculates the factorial for an integer between 0 and 20. It uses a recursive CTE to generate the result set step by step until the base case is reached.

Conclusion

Recursion is a powerful concept in SQL Server that allows for elegant and efficient solutions to complex problems. Whether it’s through recursive stored procedures, recursive functions, or recursive CTEs, recursion can simplify development and provide faster solutions for certain problems. However, it should be used with caution as it can have performance and resource implications.

By understanding recursion and its implementations in SQL Server, developers can leverage this powerful tool to solve a wide range of problems efficiently.