The User Defined Functions in SQL Server are like functions in any other programming language that accepts the parameters, performing complex calculations, and returning the result value.
Types of Functions in SQL Server
There are two types of functions in SQL Server:
- System Functions: All the built-in functions supported by the Server called System functions in SQL Server. We don’t have to bother about the logic inside them because they cannot be modified. For example, Mathematical Functions, Ranking Functions, String Functions, etc.
- User Defined Functions: SQL Server allows us to create our functions called as user defined functions in SQL Server. For example, if we want to perform some complex calculations, then we can place them in a separate function, and store it in the database. Whenever we need the calculation, we can call it. There are two types of SQL user defined functions:
- Scalar Function: It is a function that returns a single value. Generally, we have to define the function body between BEGIN … END block, but for inline scalar function, you can omit them. We can use any SQL data type as the return type except text, image, ntext, cursor, and timestamp.
- Table Valued Functions: It is a user defined function in SQL Server that returns a table.
- Inline Table valued Functions: This function returns a table data type based on a single SELECT Statement
Advantages of User Defined Functions in SQL Server
- User defined functions in SQL Server prevent us from writing the same logic multiple times.
- Within the Database, you can create the function once, and call it n number of times.
- SQL user defined functions reduce the compilation time of query by catching the execution plan and reusing them.
- This user defined functions can help us to separate the complex calculations from the regular query so that we can understand and debug the query quicker and better.
- It reduces the network traffic because of its cache plan
- SQL Server Functions also used in WHERE Clause as well. By this, we can limit the number of rows sent to the client.
Syntax of User Defined Functions in SQL Server
The syntax the SQL Server User defined functions or UDF is
CREATE FUNCTION Function_Name(@Parameter_Name Data_type, .... @Parameter_Name Data_type ) RETURNS Data_Type AS BEGIN -- Function Body RETURN Data END
- Return_Type:
- Data Type: Please specify the data type of return value. For example, VARCHAR, INT, FLOAT, etc.
- Data: Please specify the return value, and it should match the Data Type. It can be a single value or Table
- Function_Name: You can specify any name you wish to give other than the system reserved keywords. Please try to use meaningful names so that you can identify them easily.
- @Parameter_Name: Every function accepts zero or more parameters; it completely depends upon the user requirements. While declaring the parameters don’t forget the appropriate data type. For example (@name VARCHAR(50), @number INT)
- Function Body: Any query, or any complex mathematical calculations you want to implement in this particular function.
Let us see how to create or write different types of User Defined Functions in SQL Server with example.
For this SQL UDF demonstration, We are going to use [MyEmployee table] and [Department] table present in our [SQL Tutorial] Database.
TIP: Please refer to Alter User Defined Functions in SQL Server article to understand, How to Rename, Modify, or Delete SQL Server UDFs.
From the below figure you can observe that [MyEmployee table] table have fourteen records

And [Department] table has eight records.

Create SQL Scalar Function example
The Scalar User defined functions in SQL Server are very useful when you want to return a single value as the resultant. For example, total sales, or total investments, total loss, or total expenditure etc.
Create SQL Scalar Function with No parameters example
In this simple SQL scalar function example, we will show you how to create the Scalar function without any parameters.
From the below query, you can observe that we are summing the Yearly Income of the MyEmployee table.
-- SQL User Defined Functions - SQL Scalar Function example CREATE FUNCTION NoParameters () RETURNS INT AS BEGIN RETURN (SELECT SUM([YearlyIncome]) FROM [MyEmployees Table]) END

Let me show you, How it looks in the SQL Management Studio

Let us see the Output of the SQL scalar function
SELECT [EmployeeID] ,[FirstName] ,[LastName] ,[Education] ,[Occupation] ,[YearlyIncome] ,dbo.NoParameters() AS [Average Income] ,[Sales] ,[HireDate] FROM [MyEmployees Table]

Create SQL Scalar Function with parameters example
In this example we will show you, How to create SQL Scalar function with parameters. From the below query you can observe that, we are concating First name and Last Name.
NOTE: We are using the SPACE function to provide the space between the First name and last name.
-- SQL User Defined Functions - Scalar Functions example CREATE FUNCTION fullName (@firstName VARCHAR(50), @lastName VARCHAR(50)) RETURNS VARCHAR(200) AS BEGIN RETURN (SELECT @firstName + SPACE(2) + @lastName ) END
Let us see the Output by using the below query
USE [SQL Tutorial] GO SELECT [EmployeeID] -- Passing Parameters to fullname Function ,dbo.fullname([FirstName], [lastName]) AS [Name] ,[Education] ,[Occupation] ,[YearlyIncome] ,[Sales] ,[HireDate] FROM [MyEmployees Table]

Create SQL Scalar Function Where Clause example
The following SQL scalar function where clause example will accept the varchar as the parameter. And it finds the sum of the Sales amount, whose Occupation is equal to the parameter that we pass.
-- SQL User Defined Functions - SQL Scalar Function example CREATE FUNCTION average (@Occupation VARCHAR(50)) RETURNS FLOAT AS BEGIN RETURN (SELECT SUM([Sales]) FROM [MyEmployees Table] WHERE [Occupation] = @Occupation) END
Let us see the Output
USE [SQL Tutorial] GO SELECT [Occupation] ,SUM([YearlyIncome]) as [Total Income] ,SUM([Sales]) AS [Total Sale] ,dbo.average([Occupation]) AS [Total Sale from Function] FROM [MyEmployees Table] group by [Occupation]

Use SQL Scalar Function in Where Clause example
In this example, we show you how to use the SQL Scalar function in WHERE Clause
-- SQL User Defined Functions - SQL Scalar Function example USE [SQL Tutorial] GO CREATE FUNCTION AverageSale () RETURNS FLOAT AS BEGIN RETURN (SELECT AVG([Sales]) FROM [MyEmployees Table]) END
Let us see the Output
USE [SQL Tutorial] GO SELECT [EmployeeID] ,[FirstName] ,[LastName] ,[Education] ,[Occupation] ,[YearlyIncome] ,[Sales] ,[HireDate] FROM [MyEmployees Table] WHERE [Sales] >= dbo.AverageSale()

Create SQL Server Inline Table valued Functions examples
The SQL Server Inline function returns a table data type as the return value based on a single SELECT Statement
Create SQL Inline Function with No parameters example
In this simple example we will show you, How to create a SQL Inline table valued function without any parameters. From the below query, you can observe that we are selecting the top 10 records from the MyEmployee table.
-- SQL User Defined Functions - Inline Functions example USE [SQL Tutorial] GO CREATE FUNCTION TopTenCustomers () RETURNS TABLE AS RETURN ( SELECT TOP 10 [FirstName] ,[LastName] ,[Education] ,[Occupation] ,[YearlyIncome] ,[Sales] ,[HireDate] FROM [MyEmployees Table] )
Let us see the Output
USE [SQL Tutorial] GO SELECT * FROM [dbo].[TopTenCustomers] () GO

SQL Inline Function with Parameters Example
This SQL Server functions example shows how to create an SQL Inline table valued function with parameters.
From the below query, you can see we are selecting the records from both the table using INNER JOIN, whose Occupation is equal to a parameter that we pass.
-- SQL User Defined Functions - Inline Functions example USE [SQL Tutorial] GO CREATE FUNCTION CustomerbyDepartment (@profession VARCHAR(50)) RETURNS TABLE AS RETURN ( SELECT [FirstName] ,[LastName] ,[Occupation] ,[Education] ,dept.DepartmentName AS Department ,[YearlyIncome] AS Income ,[Sales] FROM [MyEmployees Table] INNER JOIN Department AS dept ON Dept.[id] = [MyEmployees Table].DeptID WHERE [Occupation] = @profession )
Let us see the Output of the SQL Inline table valued function
USE [SQL Tutorial] GO SELECT * FROM [dbo].[CustomerbyDepartment] ('Management') GO

Multi-select Table valued Functions in Sql Server examples
The SQL Server Multiple select table valued functions return the tabular result set. However, unlike the inline table valued function, we can use multiple select statements inside the function body.
In this SQL Server Multi select table valued function example will show you, How to use Multiple statements in one User defined function.
-- SQL User Defined Functions - Table Valued Functions example USE [SQL Tutorial] GO CREATE FUNCTION CustomerDepartment() RETURNS @customers TABLE ( [EmployeeID] [smallint] NOT NULL, [FirstName] [nvarchar](30) NULL, [LastName] [nvarchar](40) NULL, [Education] [nvarchar](255) NULL, [Occupation] [nvarchar](255) NULL, [YearlyIncome] [float] NULL, [Sales] [float] NULL, [HireDate] [date] NULL, [DepartmentName] [VARCHAR](50) NULL ) AS BEGIN INSERT INTO @customers SELECT [EmployeeID] ,[FirstName] ,[LastName] ,[Education] ,[Occupation] ,[YearlyIncome] ,[Sales] ,[HireDate] ,dept.DepartmentName FROM [MyEmployees Table] INNER JOIN Department AS dept ON Dept.[id] = [MyEmployees Table].DeptID -- Updating the Records UPDATE @customers SET [YearlyIncome] = [YearlyIncome] + 35200 WHERE [Sales] > (SELECT AVG(Sales) FROM [MyEmployees Table]) RETURN END
ANALYSIS
Within this User Defined Functions example, First, we are creating a table variable called @customers using the following statement
@customers TABLE
Next, we are inserting the records from [MyEmployee Table], and [Department] table into @customers table variable.
INSERT INTO @customers SELECT
Next, we are updating the Yearly income of all the customers present in the @customers table variable, whose yearly income is greater than the average sales.
UPDATE @customers SET [YearlyIncome] = [YearlyIncome] + 35200 WHERE [Sales] > (SELECT AVG(Sales) FROM [MyEmployees Table])
Let us see the Output
USE [SQL Tutorial] GO SELECT * FROM [dbo].[CustomerDepartment] () GO

Multiple User Defined Functions in SQL Server
How to use multiple User defined functions in a single SELECT Statement.
USE [SQL Tutorial] GO SELECT [EmployeeID] ,dbo.fullName([FirstName], [LastName]) AS Name -- First UDF ,[Education] ,[Occupation] ,[YearlyIncome] ,[Sales] ,[HireDate] FROM [MyEmployees Table] WHERE [Sales] >= dbo.AverageSale() -- Second UDF

One UDF inside another UDF in SQL Server
In this simple Sql server UDF example, we will show you, How to nest or insert one User defined function inside another User defined function.
USE [SQL Tutorial] GO CREATE FUNCTION CustDepartment (@education VARCHAR(50)) RETURNS TABLE AS RETURN ( SELECT dbo.fullName([FirstName],[LastName]) AS NAME ,[Occupation] ,[Education] ,dept.DepartmentName AS Department ,[YearlyIncome] AS Income ,[Sales] FROM [MyEmployees Table] INNER JOIN Department AS dept ON Dept.[id] = [MyEmployees Table].DeptID WHERE [Education] = @education )
Let us see the Output of the Nested user defined functions in Sql Server
USE [SQL Tutorial] GO SELECT * FROM [dbo].[CustDepartment] ('Bachelors')

Limitations of User Defined Functions in SQL Server
The following are the list of limitations of the SQL Server User defined Functions
- We cannot use the User defined functions, shortly called as UDF in SQL Server to modify the database state.
- SQL UDF can not return multiple result sets.
- The SQL UDF does not support error handling, such as TRY..CACHE, RAISEERROR, or @ERROR
- We cannot call a Stored Procedure from SQL UDF, but we can call an extended Stored Procedure
- The SQL User Defined Functions do not support the temporary tables, but it will allow the Table variable.
- SET statements are not allowed in SQL UDFs
- The FOR XML Clause is not permitted inside the functions