How to report an error from a SQL Server user-defined function

Mastering custom error handling and avoiding confusion

-- Creating a UDF with custom error handling and a pinch of humor mixed in
CREATE FUNCTION dbo.CustomErrorExample(@InputParam INT)
RETURNS INT
AS
BEGIN
    IF @InputParam IS NULL
       RETURN CAST('If you give me nothing, I give you an error' AS INT); -- Custom error due to NULL 

    -- UDF Logic - where magic happens

    RETURN @Result; -- Returning output is a professional courtesy
END;


The astute algorithm above unveils a well-planned scenario handling invalid input. Stalking the trail of an error message, the function returns an integer rather than a null. Covertly disguised in INT’s clothing, it confuses SQL Server into thinking it's just another regular integer datatype. Like a professional poker player, SQL Server continues the game, consequentially leading to a CAST error that unravels the ruse. Jason Bourne would be proud.

Throwback to SQL Server 2008 error handling saga

Let's turn back the dial for a bit and see how SQL Server 2008 handled errors. It was a time before Thrones were gamed and avengers assembled.

-- The one where we talk about SQL Server 2008
CREATE FUNCTION dbo.LegacyErrorSignal(@InputParam INT)
RETURNS INT
AS
BEGIN
    IF @InputParam < 0
       SELECT 1/0; -- Pushing the null to 100 real quick

    -- UDF Logic - where we make things work

    RETURN @Result; -- Returns Result. What else were you expecting?
END;


Here, a simple divide by zero error was used as the alarm bell for errors. Quite like the Unbreakable Vow from our beloved Potter universe, use it cautiously. In the following chapters, a cleaner and more subtle approach replaced this relic from the past. As always, be a good Samaritan and document all its use cases. Just like our history books, let's document our code for future generations to understand.

Table-Valued Function (TVF) and the art of error handling

In the realm of Table-Valued Functions (TVFs), the custom of explicit error messages isn't native. Fear not, we have a plan.

-- TVF with explicit error reporting as a mic-drop moment
CREATE FUNCTION dbo.ErrorReportingTVF(@InputParam INT)
RETURNS @ResultTable TABLE
(
    Result INT,
    ErrorFlag BIT,
    ErrorMessage VARCHAR(255)
)
AS
BEGIN
    IF @InputParam IS NULL
    BEGIN
        INSERT INTO @ResultTable (Result, ErrorFlag, ErrorMessage)
        VALUES (NULL, 1, 'Did you just feed me a NULL value!');
        RETURN;
    END

    -- TVF Logic - where we brew the perfect potion

    -- Populate result set, because sharing is caring
    INSERT INTO @ResultTable (Result, ErrorFlag, ErrorMessage)
    VALUES (@CalculatedResult, 0, NULL);

    RETURN;
END;


The TVFs return a result set with bonus attributes: ErrorFlag and ErrorMessage. A nonzero ErrorFlag and a populated ErrorMessage are like semaphores on a railway track, signaling an impending error.

Wise man says: validate inputs before errors become issues

In the UDF universe, invalid inputs can sometimes lead to chaos (and endless cups of coffee in some cases). Handle these inputs carefully to keep the generic errors away. Unconventional errors might just save the day by making debugging a lot easier.

Say no to 'Cryptic Mode': Make function behavior clear

Be the Good Samaritan: document the specific behaviors of your functions. This assists other developers to understand the expected behavior and why NULLs, the drama queens of SQL, are making appearances.

Endorse Developer Comfort: Assist in diagnosing NULL

Invest in developer experience (DX) and make friends with fellow coders. Offering a contextual error handle helps them understand potential pitfalls caused by NULL outcomes.

Consider alternate exit strategies apart from Divide by Zero

SELECT 1/0; might be a clever ploy, but remember it's not the only trick in the bag. Alternate error handling methods include custom return values, conditional error messages, or even table-valued functions specifically coded to handle errors.