User Defined Functions (UDFs) are a powerful feature in SQL Server that allow you to encapsulate reusable logic and perform calculations within queries. However, it is important to be aware of their limitations in order to make informed decisions when choosing between UDFs and other SQL Server objects, such as Stored Procedures.

No Access to Structural and Permanent Tables

One of the main limitations of UDFs is that they do not have access to structural and permanent tables. While UDFs can call Extended Stored Procedures, which can have access to these tables, they themselves cannot directly interact with them. This can be a significant limitation when you need to perform complex operations involving multiple tables.

Limited Number of Input Parameters

UDFs have a maximum limit of 1023 input parameters, whereas Stored Procedures can have up to 21000 input parameters. This limitation can be problematic when you have a large number of parameters that need to be passed to a UDF.

Prohibition of Non-Deterministic Built-in Functions

UDFs prohibit the usage of non-deterministic built-in functions such as GETDATE(). These functions can be used in Stored Procedures, but not in UDFs. This limitation can restrict the functionality and flexibility of UDFs in certain scenarios.

Single Result Set or Output Parameter

UDFs can only return a single result set or output parameter. While this makes them suitable for use in SELECT statements, it prevents them from returning multiple result sets like Stored Procedures. This limitation can be a hindrance when you need to perform complex data manipulations and return multiple sets of data.

No Ability to Call Stored Procedures or Execute Dynamic SQL

UDFs are limited in their ability to call Stored Procedures or execute dynamic SQL. They can only access Extended Stored Procedures, which can be a limitation when you need to perform certain operations that require the use of Stored Procedures or dynamic SQL.

Prohibition of Returning XML

UDFs do not support the FOR XML clause, which means they cannot return XML data. This limitation can be problematic when you need to generate XML output from a UDF.

Lack of Support for SET Options and Error Handling

UDFs do not support SET options that can change server-level or transaction-level settings, such as SET ROWCOUNT. Additionally, UDFs do not support error handling using RAISERROR or @@ERROR. These limitations can make it challenging to handle errors and control the behavior of UDFs in certain situations.

Despite these limitations, UDFs still have their advantages and are commonly used for repetitive tasks and modularizing frequently used code within SELECT statements. It is important to carefully consider the limitations and requirements of your specific use case when deciding whether to use UDFs or other SQL Server objects.