title: i am tryen to implement this code f sql but stil get me an error i really dont know why
body: <pre><code>create table student (
  student_ID number(6) primary key,
  student_name varchar (25) unique ,
  address varchar ,
  sex char, 
  manager_ID number(6) REFERENCES maneger (manager_ID) , 
  birth_date date NOT NULL 
  );
  create table Classes (
  class_ID number(6) primary key ,
  student_ID number (6)REFERENCES student (student_ID), FOREIGN KEY
  );
  create table manager (
  manager_ID number (6) primary key,
  manager_name char ,
  address varchar,
  Teacher_ID number (6) FOREIGN KEY, REFERENCES teacher (Teacher_ID),
  sex char
  );
  create table teacher (
  Teacher_ID number (6), primary key,
  teacher_name varchar, 
  address varchar , 
  class_ID number (6)  FOREIGN KEY REFERENCES Classes (class_ID) ,
  subject_name varchar REFERENCES subject (subject_name)
  );
  create table subject (
  subject_name varchar  primary key,
  marks number 
  );
  create learn (
  student_ID number (6) REFERENCES student (student_ID), 
  subject_name varchar REFERENCES subject (subject_name) , 
  constraint PK_student primary key (student_ID,subject_name)
  );
  create table teach (
  student_ID number (6)REFERENCES student (student_ID) , 
  Teacher_ID number (6) REFERENCES teacher (Teacher_ID),
  constraint PK_teach primary key (student_ID,Teacher_ID)
  );
  </code></pre>