我在java spring中有一个用于学习的数据库：

如果员工没有借来的书，所有的工作。
如果员工有2本书（或更多），并且该员工注册了任何课程，那么我在同一课程中看到该员工2次（如果他有3本书，那么3次，4次，等等）。

如果我使用查询“select*from registration”就可以了

github上的项目-https://github.com/katzzer/sqldemoapplicationnewversion
显示数据库数据的控制器

@GetMapping("/employees/courseDetail")
    public String getCourses(@RequestParam("courseId") long id, Model model) {
        Optional<Course> courseOptional = courseService.getCourse(id);
        if (courseOptional.isEmpty()) {
            return "redirect:/employees/courses/";
        }
        Course course = courseOptional.get();
        model.addAttribute("course", course);
        List<Employee> employees = employeeService.getAllEmployees();
        model.addAttribute("employees", employees);
        List<Employee> enrolledEmployees = course.getEmployees();
        model.addAttribute("enrolledEmployees", enrolledEmployees);

        return "CourseDetailPage";
    }

员工类别：

@Entity(name = "Employee")
@Table(name = "employee")
@Transactional
public class Employee {

    @Id
    @SequenceGenerator(
            name = "employee_sequence",
            sequenceName = "employee_sequence",
            allocationSize = 1
    )
    @GeneratedValue(
            strategy = GenerationType.SEQUENCE,
            generator = "employee_sequence"
    )
    @Column(name ="id")
    private Long id;

    @Column(
            name = "name",
            nullable = false,
            columnDefinition = "TEXT"
    )
    private String name;

    @Column(
            name = "city",
            nullable = false,
            columnDefinition = "TEXT"
    )
    private String city;

    @Column(
            name = "language",
            nullable = false,
            columnDefinition = "TEXT"
    )
    private String language;

    @OneToOne(
            mappedBy = "employee",
            orphanRemoval = true,
            cascade = {CascadeType.PERSIST, CascadeType.REMOVE}
    )
    private EmployeeIdCard employeeIdCard;

    @OneToMany(
            mappedBy = "employee",
            orphanRemoval = true,
            cascade = {CascadeType.PERSIST, CascadeType.REMOVE},
            fetch = FetchType.EAGER
    )
    private List<Book> books = new ArrayList<>();

    @ManyToMany(
            cascade = {CascadeType.PERSIST, CascadeType.REMOVE},
            fetch = FetchType.EAGER
    )
    @Fetch(value = FetchMode.SUBSELECT)
    @JoinTable(
            name = "enrolment",
            joinColumns = @JoinColumn(
                    name = "employee_id",
                    foreignKey = @ForeignKey(name = "enrolment_employee_id_fk")
            ),
            inverseJoinColumns = @JoinColumn(
                    name = "course_id",
                    foreignKey = @ForeignKey(name = "enrolment_course_id_fk")   
            )
    )
    private List<Course> courses = new ArrayList<>();

    public Employee(String name, String city, String language) {
        this.name = name;
        this.city = city;
        this.language = language;
    }

    public Employee(String name, String city, String language, EmployeeIdCard employeeIdCard) {
        this.name = name;
        this.city = city;
        this.language = language;
        this.employeeIdCard = employeeIdCard;
    }

    public Employee() {

    }

    public void addBook(Book book) {
        if (!this.books.contains(book)) {
            this.books.add(book);
            book.setEmployee(this);
        }
    }

    public void removeBook(Book book) {
        if (this.books.contains(book)) {
            this.books.remove(book);
            book.setEmployee(null);
        }
    }

    public void setEmployeeIdCard(EmployeeIdCard employeeIdCard) {
        this.employeeIdCard = employeeIdCard;
    }

    public List<Book> getBooks() {
        return books;
    }

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getCity() {
        return city;
    }

    public void setCity(String city) {
        this.city = city;
    }

    public String getLanguage() {
        return language;
    }

    public void setLanguage(String language) {
        this.language = language;
    }

    public EmployeeIdCard getEmployeeIdCard() {
        return employeeIdCard;
    }

    public void setBooks(List<Book> books) {
        this.books = books;
    }

    public List<Course> getCourses() {
        return courses;
    }

    public void enrolToCourse(Course course) {
        courses.add(course);
        course.getEmployees().add(this);
    }

    public void unEnrolToCourse(Course course) {
        courses.remove(course);
        course.getEmployees().remove(this);

图书类

@Entity(name = "Book")
@Table(name = "book")
public class Book {

    @Id
    @SequenceGenerator(
            name = "book_sequence",
            sequenceName = "book_sequence",
            allocationSize = 1
    )
    @GeneratedValue(
            strategy = SEQUENCE,
            generator = "book_sequence"
    )
    @Column(
            name = "id",
            updatable = false
    )
    private Long id;

    @Column(
            name = "created_at",
            nullable = false,
            columnDefinition = "TIMESTAMP WITHOUT TIME ZONE"
    )
    private LocalDateTime createdAt;

    @Column(
            name = "book_name",
            nullable = false
    )
    private String bookName;

    @ManyToOne (fetch = FetchType.EAGER)
    @JoinColumn(
            name = "employee_id",
//            nullable = false,
            referencedColumnName = "id",
            foreignKey = @ForeignKey (
                    name = "employee_book_fk"
            )
    )
    private Employee employee;

    public Book() {
    }

    public Book(String bookName, LocalDateTime createdAt) {
        this.bookName = bookName;
        this.createdAt = createdAt;
    }

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public LocalDateTime getCreatedAt() {
        return createdAt;
    }

    public void setCreatedAt(LocalDateTime createdAt) {
        this.createdAt = createdAt;
    }

    public String getBookName() {
        return bookName;
    }

    public void setBookName(String bookName) {
        this.bookName = bookName;
    }

    public Employee getEmployee() {
        return employee;
    }

    public void setEmployee(Employee employee) {
        this.employee = employee;
    }

课程类别：

@Entity(name = "Course")
@Table(name = "course")
public class Course {

    @Id
    @SequenceGenerator(
            name = "course_sequence",
            sequenceName = "course_sequence",
            allocationSize = 1
    )
    @GeneratedValue(
            strategy = SEQUENCE,
            generator = "course_sequence"
    )
    @Column(
            name = "id",
            updatable = false
    )
    private Long id;

    @Column(
            name = "course_name",
            nullable = false,
            columnDefinition = "TEXT"
    )
    private String courseName;

    @Column(
            name = "department",
            nullable = false,
            columnDefinition = "TEXT"
    )
    private String department;

    @ManyToMany(
            mappedBy = "courses",
            fetch = FetchType.EAGER
    )
    private List<Employee> employees = new ArrayList<>();

    public Course(String courseName, String department) {
        this.courseName = courseName;
        this.department = department;
    }

    public Course() {
    }

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getCourseName() {
        return courseName;
    }

    public void setCourseName(String courseName) {
        this.courseName = courseName;
    }

    public String getDepartment() {
        return department;
    }

    public void setDepartment(String department) {
        this.department = department;
    }

    public List<Employee> getEmployees() {
        return employees;
    }

    public void setEmployees(List<Employee> employees) {
        this.employees = employees;
    }

课程服务：

@Service
public class CourseService {

    private CourseRepository courseRepository;

    @Autowired
    public CourseService(CourseRepository courseRepository) {
        this.courseRepository = courseRepository;
    }

    public void saveCourse(Course course) {
        courseRepository.save(course);
    }

    public void saveAllCourses(List<Course> courses) {
        courseRepository.saveAll(courses);
    }

    public Optional<Course> getCourse (Long id) {
        return courseRepository.findById(id);
    }

    public List<Course> getAllCourses() {
        return courseRepository.findAll();
    }

    public void deleteCourse(Long id) {
        courseRepository.deleteById(id);
    }
}

课程假设：

public interface CourseRepository extends JpaRepository<Course, Long> {
}