How to check if a function exists in a SQL database?

Question

This is what SSMS uses when you script using the DROP and CREATE option

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 

This approach to deploying changes means that you need to recreate all permissions on the object so you might consider ALTER-ing if Exists instead.

I tend to use the Information_Schema:

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

for functions, and change Routine_Type for stored procedures

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

Why not just:

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

The second argument of object_id is optional, but can help to identify the correct object. There are numerous possible values for this type argument, particularly:

• FN : Scalar function
• IF : Inline table-valued function
• TF : Table-valued-function
• FS : Assembly (CLR) scalar-function
• FT : Assembly (CLR) table-valued function

I know this thread is old but I just wanted to add this answer for those who believe it's safer to Alter than Drop and Create. The below will Alter the Function if it exists or Create it if doesn't:

  IF NOT 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' ))
       EXEC('CREATE FUNCTION [dbo].[foo]() RETURNS INT AS BEGIN RETURN 0 END')
  GO
  ALTER FUNCTION [dbo].[foo]
  AS
  ...

I've found you can use a very non verbose and straightforward approach to checking for the existence various SQL Server objects this way:

IF OBJECTPROPERTY (object_id('schemaname.scalarfuncname'), 'IsScalarFunction') = 1
IF OBJECTPROPERTY (object_id('schemaname.tablefuncname'), 'IsTableFunction') = 1
IF OBJECTPROPERTY (object_id('schemaname.procname'), 'IsProcedure') = 1

This is based on the OBJECTPROPERTY function which is available in SQL 2005+. The MSDN article can be found here.

The OBJECTPROPERTY function uses the following signature:

OBJECTPROPERTY ( id , property ) 

You pass a literal value into the property parameter, designating the type of object you are looking for. There's a massive list of values you can supply.