SQL - Problema filtrando por el nombre

USE [Academico]

GO

SET ANSI_NULLS ON

GO

SET QUOTED_IDENTIFIER ON

GO

CREATE PROCEDURE [dbo].[9_PROMEDIO_ALUMNOS_INDICANDO_ASIGNATURA]

@ASIGNATURA NVARCHAR(50),

AS

SELECT AVG(*) FROM(

 select a.DocIdent, a.apellidos, a.nombres ,s.nombre, n.Definitiva

 from dbo.Alumno a inner join dbo.Nota n

 on a.DocIdent = n.DocIdent inner join dbo.Asignatura s

 on s.CodAsig = n.codAsig

 where s.Nombre = @ASIGNATURA

)

mysql> SELECT * FROM tabla;

+--------+--------------+

| alumno | calificacion |

+--------+--------------+

| hugo   |            9 |

| hugo   |            6 |

| hugo   |           10 |

| paco   |            5 |

| paco   |            4 |

| paco   |            5 |

| luis   |           10 |

| luis   |            9 |

| luis   |           10 |

+--------+--------------+

9 rows in set (0.00 sec)

mysql> SELECT alumno, AVG(calificacion)

    -> FROM tabla

    -> GROUP BY alumno;

+--------+-------------------+

| alumno | AVG(calificacion) |

+--------+-------------------+

| hugo   |            8.3333 |

| luis   |            9.6667 |

| paco   |            4.6667 |

+--------+-------------------+

3 rows in set (0.00 sec)

mysql> SELECT AVG(calificacion)

    -> FROM tabla;

+-------------------+

| AVG(calificacion) |

+-------------------+

|            7.5556 |

+-------------------+

1 row in set (0.00 sec)

mysql> SELECT AVG(*)

    -> FROM tabla;

ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that

corresponds to your MySQL server version for the right syntax to use near '*)

FROM tabla' at line 1