Types of User-Defined Functions in SQL Server

Modern databases are complex systems containing many different types of objects. Working with them effectively is crucial not only for maintaining database performance but also for enhancing the performance of the applications that rely on them. This is a key objective for database and application developers, managers, and administrators alike.

Among the many tools available to these specialists, user-defined functions (UDFs) deserve special attention. This article will explore UDFs in SQL Server, covering their types, how they work, and common use cases. Let's dive in.

What are user-defined functions (UDFs) in SQL Server?

In SQL databases, a user-defined function (UDF) is a function created by the user and executed within the database. In other words, a user-defined function is a predefined routine that takes one or more parameters, performs a specific task, and returns either a single scalar value or a result set.

UDFs are similar to stored procedures in that they allow developers to box T-SQL code for reuse across different applications. However, unlike stored procedures, UDFs typically return a value that can be directly used in queries.

Benefits of using UDFs in SQL Server

The primary purpose of user-defined functions is to enable the reuse and sharing of code for specific tasks such as complex calculations, data transformations, or custom manipulations. The benefits of using UDFs in databases include:

• Code reusability: UDFs allow you to create code for specific tasks and reuse it wherever needed, reducing duplication and simplifying query maintenance.
• Modular programming: Functions can be modified independently of the rest of the program. Once created, a function can be called as many times as needed.
• Faster execution: UDFs cache query execution plans and reuse them for repeated calculations, eliminating the need to reparse and reoptimize code each time.
• Reduced network traffic: By filtering and manipulating data within the database, UDFs ensure that client applications only fetch the necessary results, minimizing data transfer.
• Query integration: UDFs can be used directly in SQL queries, including within SELECT, WHERE, HAVING, and ORDER BY clauses.
• Security: Access control mechanisms can be applied to UDFs, allowing administrators to manage permissions for executing or modifying them, thereby protecting sensitive operations.

Types of user-defined functions in SQL Server

In SQL Server, user-defined functions are categorized based on the type of output they return. The two main types are scalar functions, which return a single value, and table-valued functions, which return data of table types. Table-valued functions further include inline table-valued functions, which return results as a table.

Let's take a closer look at each type. We will utilize the popular SQL Server sample database, AdventureWorks2022, and dbForge Studio for SQL Server, a powerful and feature-rich IDE designed by Devart.

Scalar functions

Scalar functions are a type of UDF that take input parameters, perform calculations or operations, and return a single data value based on those parameters.

Developers often use scalar functions in SQL Server to simplify complex queries and encapsulate business logic for applications. Like other SQL functions, you can call scalar UDFs and use their return values as a column, in a WHERE clause, or any other expression. Scalar functions can be created in SQL Server using the CREATE FUNCTION statement.

The AdventureWorks2022 database includes several user-defined functions. One example is dbo.ufnGetProductStandardCost, a user-defined scalar function. This function is explicitly created in the database with CREATE FUNCTION (unlike built-in SQL Server functions) and returns a single value of type money for each call.

It accepts two input parameters:

• @ProductID - an integer identifying the product to look up.
• @OrderDate - the date for which you want the standard cost.

You can use this function anywhere a scalar expression is valid, such as in a SELECT clause, a WHERE condition, or other expressions. Let us use it in a query to list several products with their standard costs as of August 10th, 2025:

SELECT
    p.ProductID,
    p.Name,
    dbo.ufnGetProductStandardCost(p.ProductID, '2025-08-01') AS StandardCost
FROM Production.Product AS p
WHERE p.ProductID BETWEEN 715 AND 725;

Table-valued functions

Table-valued functions in SQL Server are user-defined functions that accept parameters and return a result in the form of a table. Therefore, you can use them directly in the FROM clause of a SELECT statement, treating them just like any other table or view.

Unlike stored procedures, which can return multiple result sets, a table-valued function always returns a single rowset. This design allows using table-valued functions in queries, JOINs, and other set-based operations where SQL expects a table.

Let's look at the dbo.ufnGetContactInformation function in the AdventureWorks2022 database. This user-defined table-valued function takes a single input parameter, @PersonID, which identifies a person (contact) by their integer ID. The function returns information about that person, including their PersonID, FirstName, LastName, JobTitle, and BusinessEntityType.

SELECT *
FROM dbo.ufnGetContactInformation(250);

Instead of a single value, the table-valued function returns a table with multiple columns:

Inline table-valued functions

An inline table-valued function is a user-defined function in SQL Server that returns a result set that you can use like a table in your queries. These functions are called "inline" because they don't have a traditional function body; instead, they consist of a single SELECT statement within the RETURN clause. Thus, inline table-valued functions are like parameterized views.

SQL Server's optimizer integrates the function directly into the execution plan of the calling query, which improves performance. Additionally, inline table-valued functions don't require you to declare or populate a table variable. The single SELECT statement defines the entire result, so the function avoids unnecessary procedural steps.

We have created an inline table-valued function called dbo.ufnGetProductsByMinPrice (we will explore the creation of user-defined functions further in this article). This function allows us to list products with a certain price. Let us view all products with a price higher than $3000:

SELECT *
FROM dbo.ufnGetProductsByMinPrice(3000);

This query returns all products with a ListPrice of $3000 or more:

A comparison table for user-defined functions

We have also compiled a comparison table for the three types of user-defined functions we have reviewed:

Creation of user-defined functions in SQL Server

As we've established, user-defined functions are those created by users to store and reuse SQL code that is tested and optimized for specific tasks. You make these functions using the CREATE FUNCTION command. Now, let's explore how to create different types of user-defined functions in SQL Server.

Syntax for the creation of scalar functions

The basic syntax for the creation of a scalar function is as follows:

CREATE FUNCTION FUNCTION_NAME
(@Parameter1 DATATYPE, @Parameter2 DATATYPE, @Parameter3 DATATYPE, ...,
@ParameterN DATATYPE)
RETURNS Return_Datatype
AS
BEGIN
  --Function Body
  RETURN Return_Datatype
END

Let us create a scalar function to retrieve the full name of the person.

CREATE FUNCTION dbo.ufnGetFullName(@BusinessEntityID INT)
RETURNS NVARCHAR(200)
AS
BEGIN
  DECLARE @FullName NVARCHAR(200);

  SELECT
    @FullName = FirstName + ' ' + LastName
  FROM Person.Person
  WHERE BusinessEntityID = @BusinessEntityID;

  RETURN @FullName;
END;

To execute this function, use the query below:

SELECT
  dbo.ufnGetFullName(100) AS FullName;

SQL scalar user-defined functions help us simplify complex calculations and rules that are repeatedly used in queries or applications. Most common use cases include formatting and converting values, and performing custom calculations to make query writing easier.

Syntax for the creation of table-valued functions

We can create table-valued functions as either inline table-valued functions or multi-statement table-valued functions. The syntax differs for each type.

Inline table-valued function syntax

To create an inline table-valued function, we need to use the CREATE FUNCTION statement with the RETURNS TABLE clause. The basic syntax is:

CREATE FUNCTION [schema_name.]function_name
(
    @parameter_name data_type
)
RETURNS TABLE
AS
RETURN
(
    SELECT column1, column2, ...
    FROM table_name
    WHERE condition
);

Assume we want information about all orders placed for a specific product. The inline table-based function will deliver the necessary information:

CREATE FUNCTION dbo.ufnGetProductOrders (@ProductID INT)
RETURNS TABLE
AS
  RETURN
  (
  SELECT
    sod.SalesOrderID
   ,sod.ProductID
   ,sod.OrderQty
   ,soh.OrderDate
  FROM Sales.SalesOrderDetail sod
  INNER JOIN Sales.SalesOrderHeader soh
    ON sod.SalesOrderID = soh.SalesOrderID
  WHERE sod.ProductID = @ProductID
  );

This function returns information about product orders in a table format:

SELECT *
FROM dbo.ufnGetProductOrders(770);

Multi-statement table-valued function syntax

Multi-statement table-valued functions are similar to inline functions because they return results as a table with multiple rows and columns. However, unlike inline functions, multi-statement functions require you to define the table structure explicitly. You also need to include BEGIN and END blocks in the function's body.

The syntax to create a multi-statement table-valued function is:

CREATE FUNCTION function_name (@parameter_name data_type)
RETURNS @table_variable_name TABLE 
(
    column1 data_type,
    column2 data_type,
    -- Add more columns as needed
)
AS
BEGIN
    -- Function body (contains multiple statements)

    RETURN;
END;

In our test scenario, we want to retrieve the summary of orders placed for some specific product:

CREATE FUNCTION dbo.ufnGetProductOrderSummary(@ProductID INT)
RETURNS @OrderSummary TABLE
(
    SalesOrderID INT,
    OrderDate DATETIME,
    TotalOrderQty INT
)
AS
BEGIN
    INSERT INTO @OrderSummary (SalesOrderID, OrderDate, TotalOrderQty)
    SELECT 
        soh.SalesOrderID,
        soh.OrderDate,
        SUM(sod.OrderQty) AS TotalOrderQty
    FROM Sales.SalesOrderDetail sod
    INNER JOIN Sales.SalesOrderHeader soh
        ON sod.SalesOrderID = soh.SalesOrderID
    WHERE sod.ProductID = @ProductID
    GROUP BY soh.SalesOrderID, soh.OrderDate;

    RETURN;
END;

This function returns a summary of orders for the product with ProductID 750:

SELECT * 
FROM dbo.ufnGetProductOrderSummary(750);

Multi-statement table-valued functions offer flexibility for complex calculations. Use them when you need a customized result set with a specific table structure or when you must aggregate and transform data from multiple sources before returning it.

Best practices for UDF creation

We have explored the methods of creating user-defined functions. Here, you will find additional tips on what to pay attention to when creating these functions and how to make them more convenient and better organized.

Choose the right function type

• Scalar function - for single values and small calculations.
• Inline table-valued function - for returning tables (fast, optimizer-friendly).
• Multi-statement table-valued function - for multiple steps or complex logic only.

Follow the naming and structure rules

• Clearly describe in the name what the function does (Get, Calc, Format, etc.).
• Develop and follow a consistent naming convention (dbo.ufnGetCustomerOrders, dbo.ufnGetProductOrderSummary, etc.).
• Always specify the respective schema (e.g., dbo.) in creation and usage.

Consider the query performance

• Avoid row-by-row execution in large result sets.
• Use inline table-valued functions whenever possible.
• Avoid calling user-defined functions unnecessarily in WHERE/JOIN clauses for large datasets.
• Always check the execution plans and optimize the functions before execution.

Develop the logic and minimize side effects

• Ensure only one clear responsibility for the function.
• Break down large logic into smaller functions to execute them one by one.
• Handle NULL values correctly.

Do not refer to user-defined functions when:

• The logic is simple and is only used once in a query.
• The function would add overhead, while the benefits won't compensate for that.

Besides, always debug each new function and test its performance using appropriate volumes of realistic data before implementing it in actual databases.

Debugging and managing user-defined functions with dbForge Studio for SQL Server

Debugging functions in databases is essential, but it can be tricky. There are, however, strategies and tools to make the process easier. In this article, we used dbForge Studio for SQL Server to illustrate the creation and usage of user-defined functions. The Studio is a feature-rich alternative to the default SQL Server Management Studio (SSMS) and offers a complete toolset for database development, management, data analysis, and administration both on-premises and in the cloud.

T-SQL Debugger

An integrated T-SQL Debugger enables you to find issues with your scripts, stored procedures, stored functions, and triggers through step-by-step execution and examination:

• Set breakpoints in code to pause the execution at specific lines.
• Examine the code line by line during execution.
• Monitor the current values of variables, expressions, and query results.
• Highlight the exact lines where the error occurs.

After configuring the T-SQL Debugger, select the necessary function from the list under the Programmability section in the Database Explorer pane, right-click it, and choose Step Into (or press F11):

dbForge Studio for SQL Server will debug the selected function. Note that this option is available for local servers only.

Make sure to document all known and newly detected issues for future reference.

SQL Coding Assistance

A robust SQL Editor with a complete set of options helps you write code faster and more accurately. You can write queries from scratch, edit them, and execute them directly from the Editor. Key capabilities include:

• Speed up T-SQL query writing with IntelliSense-like auto-completion.
• Validate query syntax in real time.
• Format code with flexible settings and share settings via templates.
• Reuse code fragments with a library of code snippets.

Query Profiler

dbForge Studio for SQL Server includes a Query Profiler that helps developers and DBAs detect problematic queries and optimize them for better performance. Use it to:

• Trace SQL queries and diagnose performance issues.
• View the EXPLAIN plan and session statistics.
• Compare query profiling results visually.

Always profile queries as early as possible, especially when using complex table-valued functions in large JOINs.

Query Builder

A visual Query Builder enables you to construct queries (including those planned for use in functions) without hand-typing SQL. Build queries visually using blocks and references:

• Use drag-and-drop to select tables and columns.
• Establish connections between tables visually.
• Add WHERE and HAVING conditions and construct JOINs.

All these tools make creating, testing, and optimizing user-defined functions significantly easier and more reliable.

Master UDFs for better SQL Server management

User-defined functions play a vital role in database management. They improve code modularity, boost database performance, and enable code reuse across applications, saving time on writing, testing, and optimizing code. In SQL Server databases, user-defined functions are standard, and mastering them can significantly enhance your productivity.

This article explores the scalar and table-valued types of user-defined functions, covering their features, syntax, and practical implementation. We also looked at how dbForge Studio for SQL Server can help create, optimize, and debug these functions with ease, offering more power and flexibility than SSMS and assisting with all SQL Server-related tasks.

A fully functional trial of dbForge Studio for SQL Server is available for 30 days. Download and install it to see how it can streamline your workflows and improve your daily database operations.

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