SQL Server provides a powerful feature called User Defined Functions (UDFs) that allows you to create custom functions to perform specific tasks. In this article, we will explore the concept of UDFs and discuss how they can be used to reverse a string starting from a certain position.

Let’s start by understanding the syntax of the UDF_ReverseString function:

CREATE FUNCTION UDF_ReverseString (
    @StringToReverse VARCHAR(8000),
    @StartPosition INT
)
RETURNS VARCHAR(8000)
AS
BEGIN
    IF (@StartPosition <= 0) OR (@StartPosition > LEN(@StringToReverse))
        RETURN (REVERSE(@StringToReverse))
    
    RETURN (
        STUFF(
            @StringToReverse,
            @StartPosition,
            LEN(@StringToReverse) - @StartPosition + 1,
            REVERSE(SUBSTRING(@StringToReverse, @StartPosition, LEN(@StringToReverse) - @StartPosition + 1))
        )
    )
END
GO

The UDF_ReverseString function takes two parameters: the string to be reversed and the position from where the string starts reversing. If the start position is less than or equal to 0 or greater than the length of the string, the function simply returns the reversed string using the REVERSE function. Otherwise, it uses the STUFF function to replace a portion of the string with the reversed substring.

Now, let’s see some examples of how to use the UDF_ReverseString function:

-- Reversing the string from the third position
SELECT dbo.UDF_ReverseString('forward string', 3)
-- Result: forgnirts draw

-- Reversing the entire string by passing 0 as the beginning character
SELECT dbo.UDF_ReverseString('forward string', 0)
-- Result: gnirts drawrof

-- Reversing the entire string by passing a negative number as the beginning character
SELECT dbo.UDF_ReverseString('forward string', -9)
-- Result: gnirts drawrof

-- Reversing the entire string by passing a larger number than the string length as the beginning character
SELECT dbo.UDF_ReverseString('forward string', 900)
-- Result: gnirts drawrof

It’s important to note that SQL Server already provides a built-in REVERSE function, which can be used to reverse a complete string. If you need to reverse the entire string, it is recommended to use the REVERSE function directly instead of the UDF_ReverseString function.

Here’s an example of using the REVERSE function:

SELECT FirstName, REVERSE(FirstName) AS REVERSE
FROM Person.Contact
WHERE ContactID < 5
ORDER BY FirstName;

The above query demonstrates how to use the REVERSE function to reverse the values in the FirstName column of the Person.Contact table in the AdventureWorks database.

In conclusion, User Defined Functions in SQL Server provide a flexible way to extend the functionality of the database engine. The UDF_ReverseString function we discussed in this article is just one example of how UDFs can be used to perform specific tasks. By understanding the syntax and usage of UDFs, you can leverage their power to solve complex problems in your SQL Server environment.