开发工具：STS
代码下载链接：https://github.com/theIndoorTrain/SpringBoot_Mybatis/tree/3baea10a3a1104bda815c206954b2b687511aa3d
前言：
之前我们探讨了一对一、一对多的映射关系，今天我们来讨论多对多的映射关系。
多对多，其实可以拆成多个一对多来理解。
比如：
学生-------课程----------学生选课的关系：
（1）查询某个学生所选的全部课程；
（2）查询选修某个课程的全部学生；
今天我们就来实现这个实例。
一、数据库建表：
1.student表：
2.course表：
3.student-course表：
二、查询某个学生所选的全部课程代码实现：
1.代码实现：
（1）添加Course实体：
package com.xm.pojo;
/**
* 课程实体
* @author xm
*
*/
public class Course {
private int id;
private String name;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
Course.java
（2）添加StudntCourse实体：
package com.xm.pojo;
import java.util.List;
/**
* 学生选课实体
* @author xm
*
*/
public class StudentCourse {
private int sid;
private int cid;
private int sorce;
private List<Student> students;
private List<Course> courses;
public int getSid() {
return sid;
}
public void setSid(int sid) {
this.sid = sid;
}
public int getCid() {
return cid;
}
public void setCid(int cid) {
this.cid = cid;
}
public int getSorce() {
return sorce;
}
public void setSorce(int sorce) {
this.sorce = sorce;
}
public List<Student> getStudents() {
return students;
}
public void setStudents(List<Student> students) {
this.students = students;
}
public List<Course> getCourses() {
return courses;
}
public void setCourses(List<Course> courses) {
this.courses = courses;
}
}
StudentCourse.java
（3）在Studnent实体中添加StudentCourse列表：
package com.xm.pojo;
import java.util.List;
/**
* name:学生实体
* @author xxm
*
*/
public class Student {
/**
* content:主键id
*/
private int id;
/**
* content:姓名
*/
private String name;
private List<Book> books;
private List<StudentCourse> studentCourses;
public Student() {
// TODO Auto-generated constructor stub
}
public List<StudentCourse> getStudentCourses() {
return studentCourses;
}
public void setStudentCourses(List<StudentCourse> studentCourses) {
this.studentCourses = studentCourses;
}
public List<Book> getBooks() {
return books;
}
public void setBooks(List<Book> books) {
this.books = books;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
Student.java
（4）在数据库操作接口中添加方法：
package com.xm.mapper;
import java.util.List;
import com.xm.pojo.Student;
public interface StudentMapper {
/***********/
/**
* 根据学生id查询该学生选修的所有课程
* @param id
* @return
*/
public Student selectCourseById(Integer id);
}
StudentMapper.java
（5）完善mapper映射：
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd" >
<mapper namespace="com.xm.mapper.StudentMapper">
<resultMap type="student" id="studentMap">
<id property="id" column="id"/>
<result property="name" column="name"/>
</resultMap>
<resultMap type="student" id="courseMap" extends="studentMap">
<collection property="studentCourses" ofType="studentCourse">
<result property="sorce" column="sorce"/>
<collection property="courses" ofType="course">
<id property="id" column="cid"/>
<result property="name" column="cname"/>
</collection>
</collection>
</resultMap>
<!-- 根据学生id查询该学生选修的所有课程 -->
<select id="selectCourseById" parameterType="int" resultMap="courseMap" >
select a.*,b.sorce,c.id cid,c.name cname from student a,student_course b,course c where a.id=b.sid and b.cid=c.id and a.id=#{id}
</select>
</mapper>
StudentMapper.xml
（6）在controller中实现：
package com.xm.controller;
import java.util.List;
import javax.websocket.server.PathParam;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.DeleteMapping;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.PutMapping;
import org.springframework.web.bind.annotation.RestController;
import com.xm.mapper.StudentMapper;
import com.xm.pojo.Student;
@RestController
public class StudentController {
@Autowired
private StudentMapper studentMapper;
/************/
/**
* 根据学生id查询该学生选修的所有课程
* @param id
* @return
*/
@GetMapping("/student/course/{id}")
public Student selectCourseById(@PathVariable("id") Integer id) {
Student student = studentMapper.selectCourseById(id);
return student;
}
}
StudentController.java
2.测试结果：
（1）数据库运行
2.postman运行：
三、查询选修某个课程的全部学生代码实现：
1.代码实现：
（1）Course实体中添加StudentCourse列表：
package com.xm.pojo;
import java.util.List;
/**
* 课程实体
* @author xm
*
*/
public class Course {
private int id;
private String name;
private List<StudentCourse> studentCourses;
public List<StudentCourse> getStudentCourses() {
return studentCourses;
}
public void setStudentCourses(List<StudentCourse> studentCourses) {
this.studentCourses = studentCourses;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
Course.java
（2）添加Course数据操作接口：
package com.xm.mapper;
import com.xm.pojo.Course;
public interface CourseMapper {
/**
* 根据课程id查询选修此课程的全部学生
* @param id
* @return
*/
public Course selectStudentById(Integer id);
}
CourseMapper.java
（3）添加mapper映射：
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd" >
<mapper namespace="com.xm.mapper.CourseMapper">
<resultMap type="course" id="courseMap">
<id property="id" column="id"/>
<result property="name" column="name"/>
</resultMap>
<resultMap type="course" id="studentMap" extends="courseMap">
<collection property="studentCourses" ofType="studentCourse">
<result property="sorce" column="sorce"/>
<collection property="students" ofType="student">
<id property="id" column="sid"/>
<result property="name" column="sname"/>
</collection>
</collection>
</resultMap>
<!-- 根据课程id查询选修此课程的全部学生 -->
<select id="selectStudentById" parameterType="int" resultMap="studentMap">
select a.*,b.sorce,c.id sid,c.name sname from student c,student_course b,course a where a.id=b.cid and b.sid=c.id and a.id=#{id}
</select>
</mapper>
CourseMapper.xml
（4）添加controller：
package com.xm.controller;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RestController;
import com.xm.mapper.CourseMapper;
import com.xm.pojo.Course;
/**
* 课程
* @author xm
*
*/
@RestController
public class CourseController {
@Autowired
private CourseMapper courseMapper;
/**
* 根据课程id查询选修此课程的全部学生
* @param id
* @return
*/
@GetMapping("/course/student/{id}")
public Course selectStudentById(@PathVariable("id")Integer id) {
Course course = courseMapper.selectStudentById(id);
return course;
}
}
CourseController.java
2.测试结果：
（1）数据库运行
（2）postman运行
