用java代码+MySQL代码实现上述问题

好的，下面是一个简单的Java代码实现学生选课系统的示例，使用了Java JDBC连接MySQL数据库：

1. 首先需要创建数据库和数据表，可以使用以下SQL语句：

CREATE DATABASE `course_selection` CHARACTER SET utf8 COLLATE utf8_general_ci;

USE `course_selection`;

CREATE TABLE `student` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(50) NOT NULL,
  `gender` varchar(10) NOT NULL,
  `age` int(11) NOT NULL,
  `major` varchar(50) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;

CREATE TABLE `course` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(50) NOT NULL,
  `teacher` varchar(50) NOT NULL,
  `description` varchar(255) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;

CREATE TABLE `teacher` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(50) NOT NULL,
  `gender` varchar(10) NOT NULL,
  `age` int(11) NOT NULL,
  `course` varchar(50) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;

CREATE TABLE `course_selection` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `student_id` int(11) NOT NULL,
  `course_id` int(11) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `student_id` (`student_id`),
  KEY `course_id` (`course_id`),
  CONSTRAINT `course_selection_ibfk_1` FOREIGN KEY (`student_id`) REFERENCES `student` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
  CONSTRAINT `course_selection_ibfk_2` FOREIGN KEY (`course_id`) REFERENCES `course` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;

2. 然后创建Java项目，引入MySQL连接驱动jar包，可以使用以下代码连接MySQL数据库：

public class DatabaseUtil {
    private static final String URL = "jdbc:mysql://localhost:3306/course_selection?useUnicode=true&characterEncoding=utf8&serverTimezone=GMT%2B8&useSSL=false";
    private static final String USERNAME = "root";
    private static final String PASSWORD = "123456";
    private static Connection conn = null;

    static {
        try {
            Class.forName("com.mysql.cj.jdbc.Driver");
            conn = DriverManager.getConnection(URL, USERNAME, PASSWORD);
        } catch (ClassNotFoundException | SQLException e) {
            e.printStackTrace();
        }
    }

    public static Connection getConnection() {
        return conn;
    }
}

3. 接下来就可以实现各个功能模块的Java代码了，例如：

学生信息管理：

public class StudentDao {
    public void add(Student student) {
        Connection conn = DatabaseUtil.getConnection();
        PreparedStatement ps = null;
        try {
            String sql = "INSERT INTO student(name, gender, age, major) VALUES (?, ?, ?, ?)";
            ps = conn.prepareStatement(sql);
            ps.setString(1, student.getName());
            ps.setString(2, student.getGender());
            ps.setInt(3, student.getAge());
            ps.setString(4, student.getMajor());
            ps.executeUpdate();
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                if (ps != null) {
                    ps.close();
                }
                if (conn != null) {
                    conn.close();
                }
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }

    public void delete(int id) {
        Connection conn = DatabaseUtil.getConnection();
        PreparedStatement ps = null;
        try {
            String sql = "DELETE FROM student WHERE id=?";
            ps = conn.prepareStatement(sql);
            ps.setInt(1, id);
            ps.executeUpdate();
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                if (ps != null) {
                    ps.close();
                }
                if (conn != null) {
                    conn.close();
                }
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }

    public void update(Student student) {
        Connection conn = DatabaseUtil.getConnection();
        PreparedStatement ps = null;
        try {
            String sql = "UPDATE student SET name=?, gender=?, age=?, major=? WHERE id=?";
            ps = conn.prepareStatement(sql);
            ps.setString(1, student.getName());
            ps.setString(2, student.getGender());
            ps.setInt(3, student.getAge());
            ps.setString(4, student.getMajor());
            ps.setInt(5, student.getId());
            ps.executeUpdate();
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                if (ps != null) {
                    ps.close();
                }
                if (conn != null) {
                    conn.close();
                }
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }

    public List<Student> queryAll() {
        List<Student> list = new ArrayList<>();
        Connection conn = DatabaseUtil.getConnection();
        PreparedStatement ps = null;
        ResultSet rs = null;
        try {
            String sql = "SELECT * FROM student";
            ps = conn.prepareStatement(sql);
            rs = ps.executeQuery();
            while (rs.next()) {
                Student student = new Student();
                student.setId(rs.getInt("id"));
                student.setName(rs.getString("name"));
                student.setGender(rs.getString("gender"));
                student.setAge(rs.getInt("age"));
                student.setMajor(rs.getString("major"));
                list.add(student);
            }
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                if (rs != null) {
                    rs.close();
                }
                if (ps != null) {
                    ps.close();
                }
                if (conn != null) {
                    conn.close();
                }
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
        return list;
    }
}

选课管理：

public class CourseSelectionDao {
    public void add(CourseSelection courseSelection) {
        Connection conn = DatabaseUtil.getConnection();
        PreparedStatement ps = null;
        try {
            String sql = "INSERT INTO course_selection(student_id, course_id) VALUES (?, ?)";
            ps = conn.prepareStatement(sql);
            ps.setInt(1, courseSelection.getStudentId());
            ps.setInt(2, courseSelection.getCourseId());
            ps.executeUpdate();
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                if (ps != null) {
                    ps.close();
                }
                if (conn != null) {
                    conn.close();
                }
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }

    public void delete(int id) {
        Connection conn = DatabaseUtil.getConnection();
        PreparedStatement ps = null;
        try {
            String sql = "DELETE FROM course_selection WHERE id=?";
            ps = conn.prepareStatement(sql);
            ps.setInt(1, id);
            ps.executeUpdate();
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                if (ps != null) {
                    ps.close();
                }
                if (conn != null) {
                    conn.close();
                }
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }

    public List<CourseSelection> queryAll() {
        List<CourseSelection> list = new ArrayList<>();
        Connection conn = DatabaseUtil.getConnection();
        PreparedStatement ps = null;
        ResultSet rs = null;
        try {
            String sql = "SELECT * FROM course_selection";
            ps = conn.prepareStatement(sql);
            rs = ps.executeQuery();
            while (rs.next()) {
                CourseSelection courseSelection = new CourseSelection();
                courseSelection.setId(rs.getInt("id"));
                courseSelection.setStudentId(rs.getInt("student_id"));
                courseSelection.setCourseId(rs.getInt("course_id"));
                list.add(courseSelection);
            }
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                if (rs != null) {
                    rs.close();
                }
                if (ps != null) {
                    ps.close();
                }
                if (conn != null) {
                    conn.close();
                }
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
        return list;
    }
}

成绩管理：

public class ScoreDao {
    public void add(Score score) {
        Connection conn = DatabaseUtil.getConnection();
        PreparedStatement ps = null;
        try {
            String sql = "INSERT INTO score(student_id, course_id, score) VALUES (?, ?, ?)";
            ps = conn.prepareStatement(sql);
            ps.setInt(1, score.getStudentId());
            ps.setInt(2, score.getCourseId());
            ps.setInt(3, score.getScore());
            ps.executeUpdate();
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                if (ps != null) {
                    ps.close();
                }
                if (conn != null) {
                    conn.close();
                }
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }

    public void update(Score score) {
        Connection conn = DatabaseUtil.getConnection();
        PreparedStatement ps = null;
        try {
            String sql = "UPDATE score SET score=? WHERE student_id=? AND course_id=?";
            ps = conn.prepareStatement(sql);
            ps.setInt(1, score.getScore());
            ps.setInt(2, score.getStudentId());
            ps.setInt(3, score.getCourseId());
            ps.executeUpdate();
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                if (ps != null) {
                    ps.close();
                }
                if (conn != null) {
                    conn.close();
                }
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }

    public List<Score> queryAll() {
        List<Score> list = new ArrayList<>();
        Connection conn = DatabaseUtil.getConnection();
        PreparedStatement ps = null;
        ResultSet rs = null;
        try {
            String sql = "SELECT * FROM score";
            ps = conn.prepareStatement(sql);
            rs = ps.executeQuery();
            while (rs.next()) {
                Score score = new Score();
                score.setId(rs.getInt("id"));
                score.setStudentId(rs.getInt("student_id"));
                score.setCourseId(rs.getInt("course_id"));
                score.setScore(rs.getInt("score"));
                list.add(score);
            }
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                if (rs != null) {
                    rs.close();
                }
                if (ps != null) {
                    ps.close();
                }
                if (conn != null) {
                    conn.close