有没有更好的方法来编写avg过程？

Question

我编写了此过程，但始终显示“警告：过程创建时出现编译错误”，我不知道为什么，这是我创建的表

create table EnrolledInClasses (
    St_Id char(9) primary key, 
    C_Id char(6),
    GradeN Number(2),
    constraint FK_StId Foreign key (St_Id) references Student (St_Id),
    constraint FK_CoID Foreign key (C_Id) references course (C_Id)
    constraint CheckGrade check (Grade>-1)
);

这是步骤：

CREATE OR REPLACE PROCEDURE Avg_grades
IS
avg_grades NUMBER := 0;

BEGIN
SELECT AVG (Grade)
INTO avg_grades
FROM EnrolledInClasses
dbms_output.put_line('The average of grades is :'||avg_grades);
EXCEPTION
WHEN NO_DATA_FOUND
  THEN
  DBMS_OUTPUT.put_line ('No Data Is Found..');
  END;
  /

Answer 1