Effective Methods to Check Function Existence in SQL Server

Introduction

In SQL Server database development, it is often necessary to check if a specific function exists to avoid errors during recreation. For instance, when employing a DROP and CREATE strategy, ensuring the function is absent before creation enhances script robustness.

Best Method: Using the sys.objects View

Based on the top answer, the recommended approach is to use the sys.objects system view to check for functions. This method is adopted by SQL Server Management Studio (SSMS) in script generation, ensuring compatibility and efficiency. Example code is as follows:

IF EXISTS (SELECT *
           FROM   sys.objects
           WHERE  object_id = OBJECT_ID(N'[dbo].[foo]')
                  AND type IN ( N'FN', N'IF', N'TF', N'FS', N'FT' ))
  DROP FUNCTION [dbo].[foo]
GO

Here, type IN ( N'FN', N'IF', N'TF', N'FS', N'FT' ) specifies function types, including scalar functions (FN), inline table-valued functions (IF), table-valued functions (TF), and CLR functions (FS and FT). This method queries the system catalog views directly, offering high performance and precise matching.

Other Methods

Beyond the best method, alternative approaches are available for reference. For example, using the Information_schema.Routines view:

IF EXISTS ( SELECT  1
            FROM    Information_schema.Routines
            WHERE   Specific_schema = 'dbo'
                    AND specific_name = 'Foo'
                    AND Routine_Type = 'FUNCTION' )

This approach relies on the information schema, which may be more intuitive but potentially less performant than system views. Additionally, using the object_id function provides a concise alternative:

IF object_id('YourFunctionName', 'FN') IS NOT NULL
BEGIN
    DROP FUNCTION [dbo].[YourFunctionName]
END
GO

Here, the second parameter specifies the object type, such as 'FN' for scalar functions. This leverages built-in functions for simpler code.

Analysis and Recommendations

The best method using sys.objects view is highly reliable and performant due to its integration with SQL Server internals. However, a drawback of the DROP and CREATE strategy is the need to reassign object permissions. In some cases, consider using ALTER statements to avoid permission loss. Other methods like Information_schema offer standard SQL compatibility but may not include all system-specific objects. In practice, choose the appropriate method based on specific needs, such as preferring sys.objects for deployment scripts.

Conclusion

In summary, the best practice for checking function existence in SQL Server is to use the sys.objects view with type filtering. Other methods serve as supplements, providing flexibility and compatibility. Developers should understand the trade-offs to optimize database management and script writing.