嘿，伙计们，我有一个问题，我想在甲骨文中创建一个函数，它将计算出席人数（多少学科已注册一名学生）。你能给我个提示吗？我不知道我的语法是否正确？

CREATE OR REPLACE FUNCTION num_of_attendance (id_student IN NUMBER)
RETURN NUMBER
IS total_number_of_attendance NUMBER DEFAULT 0;
BEGIN 
    WITH attendance AS
    (
        SELECT id_student, code_course, COUNT(DISTINCT code_subject)
        FROM STUDENT NATURAL INNER JOIN attendance NATURAL INNER JOIN course NATURAL INNER JOIN course_subject NATURAL INNER JOIN subject
    );
    RETURN total_number_of_attendance;
END;
/