注：程式碼已託管在GitHub上，地址是：https://github.com/Damaer/Mybatis-Learning ，專案是mybatis-14-oneself-many2one，需要自取，需要配置maven環境以及mysql環境(sql語句在resource下的test.sql中)，覺得有用可以點個小星星，小菜鳥在此Thanks~
所謂多對多查詢，就是類似於們，一個學生可以選多門課程，一門可能可以有多個學生。
我們的資料表：

與資料庫對應的實體類：
course.java,值得注意的是，toString方法裡面我們沒有加入students屬性，這是因為在student的tostring方法裡面已經加入我們的course這個類了，如果這裡加入就會死迴圈，只加一個就可以了。

import java.util.Set;

public class Course {
    private Integer cid;
    private String cname;
    private Set<Student>students;
    @Override
    public String toString() {
        return "Course [cid=" + cid + ", cname=" + cname +"]";
    }
    public Integer getCid() {
        return cid;
    }
    public
 
 void setCid(Integer cid) {
        this.cid = cid;
    }
    public String getCname() {
        return cname;
    }
    public void setCname(String cname) {
        this.cname = cname;
    }
    public Set<Student> getStudents() {
        return students;
    }
    public void setStudents(Set<Student> students) {
        this
 
.students = students;
    }
}

Student.java

import java.util.Set;

public class Student {
    private Integer sid;
    private String sname;
    private Set<Course>courses;
    public Integer getSid() {
        return sid;
    }
    public void setSid(Integer sid) {
        this.sid = sid;
    }
    public String getSname() {
        return sname;
    }
    public void setSname(String sname) {
        this.sname = sname;
    }
    public Set<Course> getCourses() {
        return courses;
    }
    public void setCourses(Set<Course> courses) {
        this.courses = courses;
    }
    @Override
    public String toString() {
        return "Student [sid=" + sid + ", sname=" + sname + ", courses="
                + courses + "]";
    }
}

定義的介面部分：

public interface IStudentDao {
    Student selectStudentById(int id);
}

mapper.xml檔案：

<?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="dao.IStudentDao">
    <resultMap type="Student" id="studentMapper">
        <id column="sid" property="sid"/>
        <result column="sname" property="sname"/>
        <collection property="courses" ofType="Course">
            <id column="cid" property="cid"/>
            <result column="cname" property="cname"/>
        </collection>
    </resultMap>
    <select id="selectStudentById" resultMap="studentMapper">
        select sid,sname,cid,cname
        from student,middle,course
        where sid=studentId and cid=courseId and sid=#{xxx}
    </select>
</mapper>

單元測試類：

public class MyTest {
  private IStudentDao dao;
  private SqlSession sqlSession;
  @Before
  public void Before(){
    sqlSession=MyBatisUtils.getSqlSession();
    dao=sqlSession.getMapper(IStudentDao.class);
  }
  @Test
  public void TestselectMinisterById(){
    Student student=dao.selectStudentById(1);
    System.out.println(student);
  }
  @After
  public void after(){
    if(sqlSession!=null){
      sqlSession.close();
    }
  }
}

結果：

[service] 2018-07-16 20:25:37,846 - dao.IStudentDao.selectStudentById -843  [main] DEBUG dao.IStudentDao.selectStudentById  - ==>  Preparing: select sid,sname,cid,cname from student,middle,course where sid=studentId and cid=courseId and sid=? 
[service] 2018-07-16 20:25:37,894 - dao.IStudentDao.selectStudentById -891  [main] DEBUG dao.IStudentDao.selectStudentById  - ==> Parameters: 1(Integer)
[service] 2018-07-16 20:25:37,935 - dao.IStudentDao.selectStudentById -932  [main] DEBUG dao.IStudentDao.selectStudentById  - <==      Total: 2
Student [sid=1, sname=Jam, courses=[Course [cid=1, cname=JAVA], Course [cid=2, cname=C++]]]