Как вызвать динамический запрос в функции Mssql?

GO
CREATE FUNCTION TopStud(@numk int)
RETURNS varchar(max)
BEGIN
	DECLARE @c int, @stud varchar(max), @tb_name varchar(max), @tb_sql varchar(max);
	SET @c = (SELECT COUNT(name) FROM tempdb.sys.tables WHERE name LIKE '##tb_'+ '[0-9]'+ '[0-9]'+ '[0-9]'+'_'+ CAST((@numk * 2 - 1) as varchar)
		OR name LIKE '##tb_'+ '[0-9]'+ '[0-9]'+ '[0-9]'+'_'+ CAST((@numk * 2) as varchar));
	WHILE @c > 0
		BEGIN
			SET @tb_name = (SELECT name FROM (SELECT name,ROW_NUMBER() over (order by name) as rnum FROM tempdb.sys.tables WHERE name LIKE '##tb_'+ '[0-9]'+ '[0-9]'+ '[0-9]'+'_'+ CAST((@numk * 2 - 1) as varchar)
			OR name LIKE '##tb_'+ '[0-9]'+ '[0-9]'+ '[0-9]'+'_'+ CAST((@numk * 2) as varchar)) as t1 WHERE rnum = @c);

			SET @tb_sql = 'SELECT DISTINCT TOP(5) CONCAT(Id, FIO, CAST(SUM(Mark)/COUNT(Mark) as varchar)) FROM Students 
			JOIN Discipline ON DGroupNum = GroupNum 
			JOIN '+ @tb_name+' ON Id = IdStud WHERE DSemestr = '+CAST((@numk * 2 - 1) as varchar)+' OR DSemestr = '+CAST((@numk * 2) as varchar)+' GROUP BY Id, FIO;'
			EXEC(@tb_sql);
			SET @c = @c - 1; 
		END;	
	RETURN @stud;
END;