ALTER FUNCTION [dbo].[udf_GetGradeAvgByNewHireID] 
        @CourseIDs VARCHAR(MAX),
	@NewHireID INT
)
RETURNS INT
AS
BEGIN

	/*
	1. The CourseIDs passed to this function will be in the form ",1,2,3,"
	2. Replace the first and last comma with () so we will have "(1,2,3)"
	3. Use this with the IN clause to filter out the right classes for these IDs
	*/	

	DECLARE @SQL VARCHAR(MAX);

	--grade average	
	DECLARE @GradeAvg INT;

	--formatting the string to be in the form (1,2,3) from ,1,2,3,4,
	SET @CourseIDs = '|' + @CourseIDs + '|';
	SET @CourseIDs = REPLACE(@CourseIDs,'|,','(');
	SET @CourseIDs = REPLACE(@CourseIDs,',|',')');

	SET @SQL = 'SELECT @GradeAvg = AVG(NHC.Class_Grade)
				FROM	
					New_Hires_Classes NHC 
					INNER JOIN Classes Cls ON NHC.Class_ID=Cls.Class_ID
				WHERE Cls.Course_ID IN ' + @CourseIDs + 
				' AND NHC.New_Hire_ID=@NewHireID';
	EXECUTE @SQL
	
	RETURN @GradeAvg

The name 'SELECT @GradeAvg = AVG(NHC.Class_Grade)
				FROM	
					New_Hires_Classes NHC 
					INNER JOIN Classes Cls ON NHC.Class_ID=Cls.Class_ID
				WHERE Cls.Course_ID IN (1,2,3,4,5,6) AND NHC.New_Hire_ID=@NewHireID' is not a valid identifier.

	SET @SQL = 'SELECT @GradeAvg = AVG(NHC.Class_Grade)
				FROM	
					New_Hires_Classes NHC 
					INNER JOIN Classes Cls ON NHC.Class_ID=Cls.Class_ID
				WHERE Cls.Course_ID IN ' + @CourseIDs + 
				' AND NHC.New_Hire_ID=' +  CAST(@NewHireID AS VARCHAR(10)) ;