php - 相对 MySQL 数据库

Question

我有一个 MySQL 数据库表，其构建如下。 班级下方的数字表示班级的“级别”，例如高，中或低，0表示不参加该班级。

我需要建立另一个表来保存每个班级的家庭作业，但我对如何构建该表有点迷茫，特别指出行的确切列必须是什么，因为每个班级都没有表示它的特定事物。

Answer 1

A properly normalized structure would have a separate table for courses, giving each an id, and another table placing students into courses by including a student id, course id, and level.

CREATE TABLE students (
  id INT NOT NULL PRIMARY KEY AUTO_INCREMENT,
  username VARCHAR(64) NOT NULL,
  password VARCHAR(64), 
  email VARCHAR(...)
  UNIQUE KEY (username)
);

CREATE TABLE courses (
  id INT NOT NULL PRIMARY KEY AUTO_INCREMENT,
  name VARCHAR(64) NOT NULL
);

/* If a record doesn't exist for a course and student, 
   the student isn't enrolled in that course. Otherwise,
   the level is defined here */
CREATE TABLE enrollments (
  id INT NOT NULL PRIMARY KEY,
  student_id INT NOT NULL,
  course_id INT NOT NULL,
  level INT NOT NULL,
  /* Each student may be enrolled only once per course */
  UNIQUE KEY (student_id, course_id),
  FOREIGN KEY (student_id) REFERENCES students (id),
  FOREIGN KEY (course_id) REFERENCES courses (id)
);

Finally then, you can create a table for assignments assigned in each course:

CREATE TABLE assignments (
  id INT NOT NULL PRIMARY KEY,
  course_id INT NOT NULL,
  description TEXT
  /*... other columns related to the assignment as necessary*/
);

And for students to complete assignments if necessary:

CREATE TABLE student_assignments (
  student_id INT NOT NULL,
  assignment_id INT NOT NULL,
  assignment_body TEXT, /* or whatever... */
  /* Or to track when completed */
  submitted_timestamp TIMESTAMP,
  PRIMARY KEY (student_id, assignment_id),
  FOREIGN KEY (assignment_id) REFERENCES assignments (id),
  FOREIGN KEY (student_id) REFERENCES students (id)
);