如何在存储库中使用Spring Boot JPA/Hibernate进行内部连接？

Question

我目前正在学习spring boot、hibernate和spring boot JPA

我为教练中心和机构开发了一个课堂应用程序。

其中，学生在一个学院注册了多个课程

学生实体类：

@Entity
@Table(name = "student")
public class Student  {

    private String name;

    private String dob;

    private String gender;

    private String address;

    private String email;

    private Integer mobile;

    private String joined;

    private Integer instID;

    @Id
    private String studentid;

    getters and setters()....
}

课程表实体类

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

    private String name;

    private String description;

    private String logo;

    private String start;

    private String end;

    private Integer fee;
    @Id
    private String courseid;


    private Integer instID;

    getters and setters();
}

已注册类别表的实体类别

public class EnrolledCourses {

    @Id
    String enrollID;

    String courseid;
    String studentid;

    Date joined;

    getters  and setters()...

}

JPA仓库

 @Repository
    public interface StudentRepository extends CrudRepository<Student, String> {
        Student findTopByInstIDOrderByStudentidDesc(int instID);
    }

@Repository
public interface CourseRepository extends CrudRepository<Course,String> {
}

@Repository
public interface EnrolledRepository extends CrudRepository<Course,String> {
}

我的需要

现在我正在检索某个学院某个课程的注册学生。。。通过使用此MySQL查询

SELECT 
`enrolled_courses.enrollID`,
`student.name`, `student.studentid`
FROM `enrolled_courses`
INNER JOIN `student`
WHERE
`enrolled_courses.studentid` = `student.studentid`
AND
`student.instID` = 13
AND
`enrolled_courses.courseid` = '13I01C' ;

现在，我需要在CourseRepository（或enrolledstudent repo）中实现这个内部连接查询

如何做到这一点？

请指引我

Answer 1

如果我们在EnrolledCourses实体中使用hibernate映射，如

@Entity
public class EnrolledCourses {

    @Id
    String enrolledId;

    @ManyToOne
    Student student;

    @ManyToOne
    Course  course;

    Date joined;

    getters  and setters()...

}

从上面的映射，而不使用任何SQL查询，你可以检索到所有的学生谁属于一个特定的课程通过使用界面中的方法

@Repository
 public interface EnrolledRepository extends CrudRepository<EnrolledCourses,String> {
     List<EnrolledCourses> findByCourse_CourseId(String courseId);
 }

如果我们在实体之间有一些关系，我们可以使用Jpa轻松检索所有字段。