Question

I am writing a function that returns a table. There are two parameters that are passed to the function and a query is built and executed and inserted into the returning table. However I am receiving this error.

Only functions and some extended stored procedures can be executed from within a function.

I would like to not use a stored procedure as this is a simple utility function. Does anyone know if this can be done. My function is coded below, it checks for dupes for a certain column within a certain table.

-- =============================================
-- AUTHOR:      JON AIREY
-- THIS FUNCTION WILL RETURN A COUNT OF HOW MANY
-- TIMES A CERTAIN COLUMN VALUE APPEARS IN A 
-- TABLE. THIS IS HELPFUL FOR FINDING DUPES.

-- THIS FUNCTION WILL ACCEPT A COLUMN NAME, TABLE
-- NAME (MUST INCLUDE SCHEMA), AND OPTIONAL
-- DATABASE TO USE. RESULTS WILL BE RETURNED AS
-- A TABLE.
-- =============================================
ALTER FUNCTION [dbo].[fn_FindDupe]
(   
-- Add the parameters for the function here
@Column     VARCHAR(MAX), 
@Table      VARCHAR(100),
@Database   VARCHAR(100)    =   ''
)
RETURNS 
@TempTable TABLE 
        ([Column] varchar(100)
        ,[Count] int)
AS
BEGIN
    DECLARE @SQL VARCHAR(MAX)
    SET @Table =    CASE
                        WHEN @Database = ''
                        THEN @Table
                        ELSE @Database + '.' + @Table
                    END

    SET @SQL =

    '   
        INSERT INTO @TempTable

        SELECT      ' + @Column + ' 
                    ,COUNT(' + @Column + ') AS CNT
        FROM        ' + @Table + '
        GROUP BY    ' + @Column + '
        ORDER BY    CNT DESC
    '

    EXEC SP_EXECUTESQL @SQL

RETURN 
END
GO

Answer 1

You can’t use dynamic sql in a udf:

This very simple: you cannot use dynamic SQL from used-defined
functions written in T-SQL. This is because you are not permitted do
anything in a UDF that could change the database state (as the UDF may
be invoked as part of a query). Since you can do anything from dynamic
SQL, including updates, it is obvious why dynamic SQL is not
permitted.

…

In SQL 2005 and later, you could implement your function as a CLR
function. Recall that all data access from the CLR is dynamic SQL.
(You are safe-guarded, so that if you perform an update operation from
your function, you will get caught.) A word of warning though: data
access from scalar UDFs can often give performance problems.