不会还有人不知道mybatis的关联查询吧？

前言：我也是花了好久的时间进行思考，该如何把这个发展的关联查询给描述出来，来今天我们就来看看这个mybatis关联查询到的是怎么样进行关联的，码字不易，如果你觉得该文章对你有用请一键三连。

1.数据库表和实体类关系分析：

数据库表之间的关系无非就是三种：一对一，一对多，多对多

1.首先需要搞懂一对一，一对多，对多之间的关系，然后数据库表是怎么表示的？利用java的面向对象思维是怎么表示的？然后就是sql查询该怎么写？然后通过mybatis关系映射标签表示他们之间的对应关系，怎么把查询出来结果集封装到相应的对象中，然后就是你自己能不能举个一对一，一对多，多对多的业务逻辑关系，自己建立数据库表，自己建立实体类对应关系，自己使用myabtis查询出对应的结果，能举出例子你才是真的学会了。记住 收藏不一定等于学会，看了学了也不一定等于学会。那么什么才交学会呢？就是自己能根据这个知识点能够想一个具体的例子并且实现，还能把这个例子给别人描述清楚，别人能懂。那么我相信，你肯定也就明白了。

一对一：

• 外键唯一：主表的主键和从表的外键（唯一），形成主外键关系，外键唯一unique。
• 外键是主键：主表的主键和从表的主键，形成主外键关系。

看个例子：比如一个学生对一张学生卡，也就是说一个学生只能有一张学生卡，而这张学生卡也只能属于这个学生。

我们来看下数据库表之间怎么表示一对一的关系

我们在来看下java里面利用面向对象的思维是怎么实现一对一的关系的。

一对多：

在多的一方创建一个字段，字段作为外键指向一的一方的主键。

举个例子：一个学生对应一个班级，一个学生只能属于某个班级，一个班级下可以有多个学生，这就是一对多的关系。

数据库表之间对应关系的表示：

利用java面向对象的方式表示一对多;

多对多：

需要创建第三张表，中间表中至少两个字段，这两个字段分别作为外键指向各自一方的主键。

例子：比如学生和老师之间的关系，一个学生可以有多个任课1老师，一个任课老师也可以有多名学生。

数据库表关系描述：

利用java的面向对象思维描述：

我这里已经把数据库表给大家准备好了，大家只需要复制粘贴进行运行就好了，注意不同版本的数据库，可能会出现异常，我这里使用的是5.7的版本，也可以根据我这里的字段类型自己去创建，利用可视化工具去创建也是非常快的。

2.数据库表准备：

t_class表

DROP TABLE IF EXISTS `t_class`;
CREATE TABLE `t_class`  (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `class_name` varchar(255) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NULL DEFAULT NULL,
  PRIMARY KEY (`id`) USING BTREE
) ENGINE = InnoDB AUTO_INCREMENT = 3 CHARACTER SET = utf8mb4 COLLATE = utf8mb4_general_ci ROW_FORMAT = Dynamic;

-- ----------------------------
-- Records of t_class
-- ----------------------------
INSERT INTO `t_class` VALUES (101, '软件工程');
INSERT INTO `t_class` VALUES (102, '云计算');

SET FOREIGN_KEY_CHECKS = 1;

t_student表：

DROP TABLE IF EXISTS `t_student`;
CREATE TABLE `t_student`  (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(255) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NULL DEFAULT NULL,
  `age` int(11) NULL DEFAULT NULL,
  `class_id` int(11) NULL DEFAULT NULL,
  PRIMARY KEY (`id`) USING BTREE,
  INDEX `class_id`(`class_id`) USING BTREE,
  CONSTRAINT `t_student_ibfk_1` FOREIGN KEY (`class_id`) REFERENCES `t_class` (`id`) ON DELETE RESTRICT ON UPDATE RESTRICT
) ENGINE = InnoDB AUTO_INCREMENT = 5 CHARACTER SET = utf8mb4 COLLATE = utf8mb4_general_ci ROW_FORMAT = Dynamic;

-- ----------------------------
-- Records of t_student
-- ----------------------------
INSERT INTO `t_student` VALUES (1, '张三', 18, 101);
INSERT INTO `t_student` VALUES (2, '李四', 19, 101);
INSERT INTO `t_student` VALUES (3, '小明', 20, 102);
INSERT INTO `t_student` VALUES (4, '王五', 16, 102);

SET FOREIGN_KEY_CHECKS = 1;

t_student_card表：

DROP TABLE IF EXISTS `t_student_card`;
CREATE TABLE `t_student_card`  (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `student_id` int(11) NULL DEFAULT NULL,
  `student_card` varchar(255) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NULL DEFAULT NULL,
  `balance` int(255) NULL DEFAULT NULL,
  PRIMARY KEY (`id`) USING BTREE,
  INDEX `student_id`(`student_id`) USING BTREE,
  CONSTRAINT `t_student_card_ibfk_1` FOREIGN KEY (`student_id`) REFERENCES `t_student` (`id`) ON DELETE RESTRICT ON UPDATE RESTRICT
) ENGINE = InnoDB AUTO_INCREMENT = 5 CHARACTER SET = utf8mb4 COLLATE = utf8mb4_general_ci ROW_FORMAT = Dynamic;

-- ----------------------------
-- Records of t_student_card
-- ----------------------------
INSERT INTO `t_student_card` VALUES (1, 1, '1001', 200);
INSERT INTO `t_student_card` VALUES (2, 2, '1002', 220);
INSERT INTO `t_student_card` VALUES (3, 3, '1003', 100);
INSERT INTO `t_student_card` VALUES (4, 4, '1004', 500);

SET FOREIGN_KEY_CHECKS = 1;

t_student_teacher表：

DROP TABLE IF EXISTS `t_student_teacher`;
CREATE TABLE `t_student_teacher`  (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `m_student_id` int(11) NULL DEFAULT NULL,
  `m_teacher_id` int(11) NULL DEFAULT NULL,
  PRIMARY KEY (`id`) USING BTREE,
  INDEX `m_student_id`(`m_student_id`) USING BTREE,
  INDEX `m_teacher_id`(`m_teacher_id`) USING BTREE,
  CONSTRAINT `t_student_teacher_ibfk_1` FOREIGN KEY (`m_student_id`) REFERENCES `t_student` (`id`) ON DELETE RESTRICT ON UPDATE RESTRICT,
  CONSTRAINT `t_student_teacher_ibfk_2` FOREIGN KEY (`m_teacher_id`) REFERENCES `t_teacher` (`id`) ON DELETE RESTRICT ON UPDATE RESTRICT
) ENGINE = InnoDB AUTO_INCREMENT = 9 CHARACTER SET = utf8mb4 COLLATE = utf8mb4_general_ci ROW_FORMAT = Dynamic;

-- ----------------------------
-- Records of t_student_teacher
-- ----------------------------
INSERT INTO `t_student_teacher` VALUES (1, 1, 1);
INSERT INTO `t_student_teacher` VALUES (2, 1, 2);
INSERT INTO `t_student_teacher` VALUES (3, 1, 3);
INSERT INTO `t_student_teacher` VALUES (4, 1, 4);
INSERT INTO `t_student_teacher` VALUES (6, 2, 2);
INSERT INTO `t_student_teacher` VALUES (7, 3, 2);
INSERT INTO `t_student_teacher` VALUES (8, 4, 2);

SET FOREIGN_KEY_CHECKS = 1;

t_teacher表：

DROP TABLE IF EXISTS `t_teacher`;
CREATE TABLE `t_teacher`  (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `teacher` varchar(255) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NOT NULL,
  PRIMARY KEY (`id`) USING BTREE
) ENGINE = InnoDB AUTO_INCREMENT = 5 CHARACTER SET = utf8mb4 COLLATE = utf8mb4_general_ci ROW_FORMAT = Dynamic;

-- ----------------------------
-- Records of t_teacher
-- ----------------------------
INSERT INTO `t_teacher` VALUES (1, 'admin');
INSERT INTO `t_teacher` VALUES (2, 'root');
INSERT INTO `t_teacher` VALUES (3, 'Tom');
INSERT INTO `t_teacher` VALUES (4, 'jack');

SET FOREIGN_KEY_CHECKS = 1;

表已经建立好了，大家可以通过以下sql进行感受下他们直接的对应关系：

一对多：

查询某个学生是属于那个班级的：
 select  s.*,c.id as cid ,c.class_name  from  t_student s left  join t_class c on s.class_id =c.id where s.id=1;
查询某个班级下有哪些学生？
select *  from t_class c join t_student s on c.id=s.class_id where c.id=101;  需要注意的是联合查询两个表中的主键字段名都为id会有问题，需要取别名

一对一：
查询某个学生的余额信息：
 select * from t_student s left join t_student_card c on s.id =c.student_id where s.id=1;

根据学生卡号查询这个余额对应的是那个学生？
select * from t_student s right join t_student_card c on s.id =c.student_id where c.student_card='1003';

多对多

查询某个学生的任教老师有哪些？
select  s.id as sid,s.name,s.age,s.class_id,t.id as tid, t.teacher from t_student s left join t_student_teacher m on s.id= m.m_student_id left join t_teacher t on m.m_teacher_id=t.id where s.id=1;


查询某个老师底下后哪些学生？
select s.id as sid,s.name,s.age,s.class_id, t.id as tid , t.teacher from t_teacher t left join t_student_teacher m on t.id=m.m_teacher_id left join t_student s on m.m_student_id=s.id where t.id=2;

因为写的比较匆忙，可能sql写的不是很好，但是能用，如果你有更好的写法，可以进行优化。

3.mybatis关系映射标签说明：

resultMap：封装返回的结果集

id：数据库表中对应的id字段

result: 数据库表中的普通字段

collections：一对多关系属性映射标签

association：一对一关系属性映射标签

property：java中实体类属性

column:数据库字段

type：java实体类的类名配合association进行使用

ofType：java实体类类名配合collections进行使用

4.具体代码实现过程：

1.实体类

Class：班级实体类

/ 班级类 对应数据库中的 班级表
public class Class {
    
    
    // 自增主键id
    private  int classId;
    // 班级名称
    private String className;

    // 建立学生表和班级表一对多的关系
    private List<Student> studentList;

    public int getClassId() {
    
    
        return classId;
    }

    public void setClassId(int classId) {
    
    
        this.classId = classId;
    }

    public String getClassName() {
    
    
        return className;
    }

    public void setClassName(String className) {
    
    
        this.className = className;
    }

    public List<Student> getStudentList() {
    
    
        return studentList;
    }

    public void setStudentList(List<Student> studentList) {
    
    
        this.studentList = studentList;
    }

    @Override
    public String toString() {
    
    
        return "Class{" +
                "classId=" + classId +
                ", className='" + className + '\'' +
                '}';
    }
}

Student：学生实体类：

// 学生类 对应数据库中学生表
public class Student {
    
    
    // 自增主键id
    private int studentId;
    // 学生姓名
    private String name;
    // 学生年龄
    private int age;
    // 学生的班级编号
    private int classId;

    // 建立学生表和学生卡表的一对一关系
    private StudentCard studentCard;

    //建立学生表和班级表一对多的关系
    private Class studentClass;

    // 建立学生表和教师表多对多关系
    private List<Teacher> teacherList;

    public List<Teacher> getTeacherList() {
    
    
        return teacherList;
    }

    public void setTeacherList(List<Teacher> teacherList) {
    
    
        this.teacherList = teacherList;
    }

    public int getStudentId() {
    
    
        return studentId;
    }

    public void setStudentId(int studentId) {
    
    
        this.studentId = studentId;
    }

    public String getName() {
    
    
        return name;
    }

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

    public int getAge() {
    
    
        return age;
    }

    public void setAge(int age) {
    
    
        this.age = age;
    }

    public int getClassId() {
    
    
        return classId;
    }

    public void setClassId(int classId) {
    
    
        this.classId = classId;
    }

    public StudentCard getStudentCard() {
    
    
        return studentCard;
    }

    public void setStudentCard(StudentCard studentCard) {
    
    
        this.studentCard = studentCard;
    }

    public Class getStudentClass() {
    
    
        return studentClass;
    }

    public void setStudentClass(Class studentClass) {
    
    
        this.studentClass = studentClass;
    }

    @Override
    public String toString() {
    
    
        return "Student{" +
                "studentId=" + studentId +
                ", name='" + name + '\'' +
                ", age=" + age +
                ", classId=" + classId +
                '}';
    }


}

StudentCard：学生卡类

// 学生卡类 对应数据库中的学生卡表
public class StudentCard {
    
    
    // 自增主键id
    private  int cardId;
    // 学生的id
    private int studentId;
    // 学生的卡号
    private String studentCard;
    // 学生卡的余额
    private int balance;

    // 建立学生卡和学生之间的一对一关系
    private Student student;

    public int getCardId() {
    
    
        return cardId;
    }

    public void setCardId(int cardId) {
    
    
        this.cardId = cardId;
    }

    public int getStudentId() {
    
    
        return studentId;
    }

    public void setStudentId(int studentId) {
    
    
        this.studentId = studentId;
    }

    public String getStudentCard() {
    
    
        return studentCard;
    }

    public void setStudentCard(String studentCard) {
    
    
        this.studentCard = studentCard;
    }

    public int getBalance() {
    
    
        return balance;
    }

    public void setBalance(int balance) {
    
    
        this.balance = balance;
    }

    public Student getStudent() {
    
    
        return student;
    }

    public void setStudent(Student student) {
    
    
        this.student = student;
    }

    @Override
    public String toString() {
    
    
        return "StudentCard{" +
                "cardId=" + cardId +
                ", studentId=" + studentId +
                ", studentCard='" + studentCard + '\'' +
                ", balance=" + balance +
                '}';
    }
}

Teacher：教师实体类

// 教师类 对应数据库中的教师表
public class Teacher {
    
    
    // 自增主键id
    private int id;
    // 教师名称
    private String teacher;

    // 建立教师表和学生表之间多对多的关系
    private List<Student> studentList;

    public int getId() {
    
    
        return id;
    }

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

    public String getTeacher() {
    
    
        return teacher;
    }

    public void setTeacher(String teacher) {
    
    
        this.teacher = teacher;
    }

    public List<Student> getStudentList() {
    
    
        return studentList;
    }

    public void setStudentList(List<Student> studentList) {
    
    
        this.studentList = studentList;
    }

    @Override
    public String toString() {
    
    
        return "Teacher{" +
                "id=" + id +
                ", teacher='" + teacher + '\'' +
                '}';
    }
}

2.dao层

classDao:一对多

public interface ClassDao {
    
    

    /**
     * 根据班级id查询出某个班级，顺便查询出这个班级里有哪些学生
     * @param classId 班级编号
     * @return
     */
    Class selectByClassId(@Param("classId") String classId);

}

---------------------------------------mapper.xml------------------------------------------------------------------
    
 <mapper namespace="com.compass.mybatis.dao.ClassDao">

    <!--  查询某个班级时，顺带查询出这个班级底下的所有学生   -->
    <select id="selectByClassId" resultMap="resultClass" parameterType="String">
        select  s.*,c.id as cid ,c.class_name  from t_class c join t_student s on c.id=s.class_id where c.id=#{
    
    classId};
    </select>

    <resultMap id="resultClass" type="Class">
        <id property="classId" column="cid"/>
        <result property="className" column="class_name"/>
        <collection property="studentList" ofType="Student">
            <id property="studentId" column="id" />
            <result property="name" column="name"/>
            <result property="age" column="age"/>
            <result property="classId" column="class_id"/>
        </collection>
    </resultMap>
    
</mapper>
    
    

StudentCardDao:一对一

public interface StudentCardDao {
    
    

    /**
     * 根据学生卡号查询出对应的是那个学生
     * @param cardId
     * @return
     */
    StudentCard selectByStudentCardId(@Param("cardId") String cardId);
}
---------------------------------------mapper.xml------------------------------------------------------------------
 <mapper namespace="com.compass.mybatis.dao.StudentCardDao">

<!--   查询学生卡信息的时候顺带查询出这个学生卡是属于那个学生的  -->
    <select id="selectByStudentCardId" resultMap="resultStudentCard" parameterType="String">
        select * from t_student s right join t_student_card c on s.id =c.student_id where c.student_card=#{
    
    cardId};
    </select>

   <resultMap id="resultStudentCard" type="StudentCard">
        <id property="studentId" column="id"/>
        <result property="studentCard" column="student_card"/>
        <result property="studentId" column="student_id"/>
        <result property="balance" column="balance"/>
        <association property="student" javaType="Student">
            <id property="studentId" column="id" />
            <result property="name" column="name"/>
            <result property="age" column="age"/>
            <result property="classId" column="class_id"/>
        </association>
    </resultMap>


</mapper>   
    

StudentDao：一对一，一对多，多对多

public interface StudentDao {
    
    

    /** 一对一
     * 查询某个学生的同时获取到他的余额信息
     * @param studentId 学生id
     * @return
     */
    Student selectByStudentId(@Param("studentId") String studentId);

    /** 一对多
     * 查询这个学生的同时 查询出他对应的班级
     * @param studentId
     * @return
     */
    Student selectByStudentIdClass(@Param("studentId") String studentId);

    /**
     * 查询某个学生的同时查询他所有的任教老师
     * @param studentId
     * @return
     */
    Student selectByIdTeacher(@Param("studentId") String studentId);

}
---------------------------------------mapper.xml------------------------------------------------------------------
 <mapper namespace="com.compass.mybatis.dao.StudentDao">


<!--  查询这个学生的时候,顺带查询出这个学生对应的学生卡信息  -->
    <select id="selectByStudentId" resultMap="resultStudentAndCard" parameterType="String">
        select * from t_student s left join t_student_card c on s.id =c.student_id where s.id=#{
    
    studentId};
    </select>


    <resultMap id="resultStudentAndCard" type="Student">
        <id property="studentId" column="id" />
        <result property="name" column="name"/>
        <result property="age" column="age"/>
        <result property="classId" column="class_id"/>
        <association property="studentCard" javaType="StudentCard">
            <id property="cardId" column="id"/>
            <result property="studentCard" column="student_card"/>
            <result property="studentId" column="student_id"/>
            <result property="balance" column="balance"/>
        </association>
    </resultMap>

<!--  查询某个学生的时候，顺带查询出他属于的是那个班级  -->
    <select id="selectByStudentIdClass" resultMap="resultStudentAndClass" parameterType="String">
        select  s.*,c.id as cid ,c.class_name  from  t_student s left  join t_class c on s.class_id =c.id where s.id=#{
    
    studentId};
    </select>

    <resultMap id="resultStudentAndClass" type="Student">
        <id property="studentId" column="id" />
        <result property="name" column="name"/>
        <result property="age" column="age"/>
        <result property="classId" column="class_id"/>
        <association property="studentClass" javaType="Class">
            <id property="classId" column="cid"/>
            <result property="className" column="class_name"/>
        </association>
    </resultMap>

<!--  查询出这个学生的同时，查询出他的任教老师有哪些  -->
    <select id="selectByIdTeacher" resultMap="resultStudentAndTeacher" parameterType="String">
        select  s.id as sid,s.name,s.age,s.class_id,t.id as tid, t.teacher from t_student s left join
            t_student_teacher m on s.id= m.m_student_id left join t_teacher t on m.m_teacher_id=t.id where s.id=#{
    
    studentId};
    </select>

    <resultMap id="resultStudentAndTeacher" type="Student">
        <id property="studentId" column="sid" />
        <result property="name" column="name"/>
        <result property="age" column="age"/>
        <result property="classId" column="class_id"/>
        <collection property="teacherList" ofType="Teacher">
            <id property="id" column="tid"/>
            <result property="teacher" column="teacher"/>
        </collection>
    </resultMap>

</mapper>   

TeacherDao:多对多

public interface TeacherDao {
    
    

    /**
     * 查询某个老师的同时查询出他底下的学生
     * @param teacherId
     * @return
     */
    Teacher selectByTeacherId(@Param("teacherId") String teacherId);

}
---------------------------------------mapper.xml------------------------------------------------------------------
    <mapper namespace="com.compass.mybatis.dao.TeacherDao">


<!--  查询某个老师的同时，查询出他教的学生有哪些   -->
    <select id="selectByTeacherId" resultMap="resultTeacher" parameterType="String">
        select s.id as sid,s.name,s.age,s.class_id, t.id as tid , t.teacher from t_teacher t left join
            t_student_teacher m on t.id=m.m_teacher_id left join t_student s on m.m_student_id=s.id where t.id=#{
    
    teacherId};
    </select>


    <resultMap id="resultTeacher" type="Teacher">
        <id property="id" column="tid"/>
        <result property="teacher" column="teacher"/>
        <collection property="studentList" ofType="Student">
            <id property="studentId" column="sid" />
            <result property="name" column="name"/>
            <result property="age" column="age"/>
            <result property="classId" column="class_id"/>
        </collection>
    </resultMap>


</mapper>

一对的另外一种映射方式：

    <resultMap id="resultStudentAndCard2" type="Student">
        <id property="studentId" column="id" />
        <result property="name" column="name"/>
        <result property="age" column="age"/>
        <result property="classId" column="class_id"/>
        <result property="studentCard.cardId" column="id"/>
        <result property="studentCard.studentCard" column="student_card"/>
        <result property="studentCard.studentId" column="student_id"/>
        <result property="studentCard.balance" column="balance"/>
    </resultMap>

一对多的另外一中方式:使用两条sql去进行完成

    <!--  使用两条SQL完成一对多查询  -->
    <!--  按照班级id查询班级信息   -->
    <select id="selectByClassId2" resultMap="resultMapClass" parameterType="String">
        select id as cid,class_name from  t_class where id=#{classId};

    </select>

    <!--   按照班级id查询学生信息  -->
    <select id="selectByStudent" resultMap="resultStudent">
        select * from t_student where class_id=#{classId};
    </select>

    <!--  封装查询出来的学生信息  -->
    <resultMap id="resultStudent" type="Student">
        <id property="studentId" column="id" />
        <result property="name" column="name"/>
        <result property="age" column="age"/>
        <result property="classId" column="class_id"/>
    </resultMap>

    <!-- 封装查询出来的班级信息 -->
    <resultMap id="resultMapClass" type="Class">
        <id property="classId" column="cid"/>
        <result property="className" column="class_name"/>
        <collection property="studentList" ofType="Student" select="selectByStudent" column="cid">
        </collection>
    </resultMap>

3.mybatis工具类

public class MyBatisUtils {
    
    

    private static SqlSessionFactory factory = null;

    static {
    
    
        //读取mybatis.xml配置文件
        String config="mybatis.xml";
        try {
    
    
            //将配置文件加入到流中
            InputStream in = Resources.getResourceAsStream(config);
            //创建factory对象
             factory = new SqlSessionFactoryBuilder().build(in);

        } catch (IOException e) {
    
    
            e.printStackTrace();
        }


    }
    public  static SqlSession getSqlSession(){
    
    
        SqlSession sqlSession = null;
        if (factory!=null){
    
    
            //如果 factory!=null 就创建 sqlSession对象
             sqlSession = factory.openSession(true);//true自动提交事务，false表示手动提交事务
        }

        return sqlSession;
    }
}

4.测试代码

public class MyTest {
    
    

   // 查询某个学生的同时获取到他的余额信息 （一对一关系）
   @Test
   void selectByStudentId(){
    
    
      SqlSession sqlSession = MyBatisUtils.getSqlSession();
      StudentDao mapper = sqlSession.getMapper(StudentDao.class);
      Student student = mapper.selectByStudentId("1");
      System.out.println("学生对象="+student);
      System.out.println("学生对象的学生卡信息="+student.getStudentCard());
   }

   // 根据卡号查询出对应的是那个学生（一对一关系）
   @Test
   void selectByStudentCardId(){
    
    
      SqlSession sqlSession = MyBatisUtils.getSqlSession();
      StudentCardDao mapper = sqlSession.getMapper(StudentCardDao.class);
      StudentCard card = mapper.selectByStudentCardId("1001");
      System.out.println("学生卡信息="+card);
      System.out.println("该学生卡是属于那个学生="+card.getStudent());
   }

   // 根据班级id查询出某个班级，顺便查询出这个班级里有哪些学生 一条SQL完成
   @Test
   void selectByClassId(){
    
    
      SqlSession sqlSession = MyBatisUtils.getSqlSession();
      ClassDao mapper = sqlSession.getMapper(ClassDao.class);
      Class className = mapper.selectByClassId("101");
      List<Student> studentList = className.getStudentList();
      System.out.println("id为101对应的班级是="+className);
      System.out.println("----------id为101对应的班级下对应的学生---------");
      System.out.println(studentList);


   }

   // 根据班级id查询出某个班级，顺便查询出这个班级里有哪些学生 使用两条SQL完成
   @Test
   void selectByClassId2(){
    
    
      SqlSession sqlSession = MyBatisUtils.getSqlSession();
      ClassDao mapper = sqlSession.getMapper(ClassDao.class);
      Class className = mapper.selectByClassId2("101");
      List<Student> studentList = className.getStudentList();
      System.out.println("id为101对应的班级是="+className);
      System.out.println("----------id为101对应的班级下对应的学生---------");
      System.out.println(studentList);


   }

   // 查询这个学生的同时 查询出他对应的班级
   @Test
   void selectByStudentIdClass(){
    
    
      SqlSession sqlSession = MyBatisUtils.getSqlSession();
      StudentDao mapper = sqlSession.getMapper(StudentDao.class);
      Student student = mapper.selectByStudentIdClass("1");
      System.out.println("学生="+student);
      System.out.println("学生对应班级="+student.getStudentClass());

   }

   // 查询某个学生的同时查询他所有的任教老师
   @Test
   void selectByIdTeacher(){
    
    
      SqlSession sqlSession = MyBatisUtils.getSqlSession();
      StudentDao mapper = sqlSession.getMapper(StudentDao.class);
      Student student = mapper.selectByIdTeacher("1");
      System.out.println("学生="+student);
      System.out.println("学生对应的老师="+student.getTeacherList());

   }


   // 查询某个老师的同时查询出他底下的学生
   @Test
   void selectByTeacherId(){
    
    
      SqlSession sqlSession = MyBatisUtils.getSqlSession();
      TeacherDao mapper = sqlSession.getMapper(TeacherDao.class);
      Teacher teacher = mapper.selectByTeacherId("2");
      System.out.println("老师="+teacher);
      System.out.println("老师底下的学生="+teacher.getStudentList());

   }

}

项目的目录结构：

entIdClass(“1”);
System.out.println(“学生=”+student);
System.out.println(“学生对应班级=”+student.getStudentClass());

}

// 查询某个学生的同时查询他所有的任教老师
@Test
void selectByIdTeacher(){
SqlSession sqlSession = MyBatisUtils.getSqlSession();
StudentDao mapper = sqlSession.getMapper(StudentDao.class);
Student student = mapper.selectByIdTeacher(“1”);
System.out.println(“学生=”+student);
System.out.println(“学生对应的老师=”+student.getTeacherList());

}

// 查询某个老师的同时查询出他底下的学生
@Test
void selectByTeacherId(){
SqlSession sqlSession = MyBatisUtils.getSqlSession();
TeacherDao mapper = sqlSession.getMapper(TeacherDao.class);
Teacher teacher = mapper.selectByTeacherId(“2”);
System.out.println(“老师=”+teacher);
System.out.println(“老师底下的学生=”+teacher.getStudentList());

}

}

项目的目录结构：

![在这里插入图片描述](https://img-blog.csdnimg.cn/20210527155707491.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L20wXzQ2MTg4Njgx,size_16,color_FFFFFF,t_70)