MyBatis框架03 实现关联查询

一、创建数据库表
（一）创建教师表
执行SQL语句，创建教师表t_teacher

CREATE TABLE `t_teacher` (
`t_id` int(11) NOT NULL AUTO_INCREMENT,
`t_name` varchar(20) DEFAULT NULL,
PRIMARY KEY (`t_id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8;

（二）创建学生表
执行SQL语句，创建学生表t_student

CREATE TABLE `t_student` (
  `s_id` int(11) NOT NULL AUTO_INCREMENT,
  `s_name` varchar(30) DEFAULT NULL,
  `s_gender` varchar(10) DEFAULT NULL,
  `s_age` int(9) DEFAULT NULL,
  `class_id` int(11) DEFAULT NULL,
  PRIMARY KEY (`s_id`)
) ENGINE=InnoDB AUTO_INCREMENT=10 DEFAULT CHARSET=utf8;

INSERT INTO `t_student` VALUES('1','佟大为','男','20','1');
INSERT INTO `t_student` VALUES('2','李芳玉','女','20','1');
INSERT INTO `t_student` VALUES('3','郑大林','男','19','2');
INSERT INTO `t_student` VALUES('4','温晓辉','男','22','1');
INSERT INTO `t_student` VALUES('5','吴文静','女','18','2');
INSERT INTO `t_student` VALUES('6','肖文艳','女','20','3');
INSERT INTO `t_student` VALUES('7','杨文军','男','23','3');
INSERT INTO `t_student` VALUES('8','唐雨涵','女','20','2');
INSERT INTO `t_student` VALUES('','金雨欣','女','20','2');
![在这里插入图片描述](https://img-blog.csdnimg.cn/20210310151909149.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3RyaWV0,size_16,color_FFFFFF,t_70)

（三）创建班级表
执行SQL语句，创建班级表t_class

CREATE TABLE `t_class` (
  `c_id` int(11) NOT NULL AUTO_INCREMENT,
  `c_name` varchar(20) DEFAULT NULL,
  `teacher_id` int(11) DEFAULT NULL,
PRIMARY KEY (`c_id`),
KEY `fk_teacher_id` (`teacher_id`),
CONSTRAINT `fk_teacher_id` FOREIGN KEY (`teacher_id`) REFERENCES `t_teacher` (`t_id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8;

INSERT INTO `t_class` VALUES ('1', '2020软件1班', '3');
INSERT INTO `t_class` VALUES ('2', '2020软件2班', '2');
INSERT INTO `t_class` VALUES ('3', '2020软件3班', '1');

二、创建与数据库表对应的实体类
（一）创建教师实体类

package net.tjl.mybatis.bean;
/**
 * 功能：教师实体类
 * 作者：谭金兰
 * 日期：2021年03月10日
 */

public class Teacher {
    
    
    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;
    }

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

（二）创建学生实体类

package net.tjl.mybatis.bean;
/**
 * 功能：学生实体类
 * 作者：谭金兰
 * 日期：2021年03月10日
 */

public class Student {
    
    
    private int id;
    private String name;
    private String gender;
    private int age;
    private Clazz clazz;

    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;
    }

    public String getGender() {
    
    
        return gender;
    }

    public void setGender(String gender) {
    
    
        this.gender = gender;
    }

    public int getAge() {
    
    
        return age;
    }

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

    public Clazz getClazz() {
    
    
        return clazz;
    }

    public void setClazz(Clazz clazz) {
    
    
        this.clazz = clazz;
    }

    @Override
    public String toString() {
    
    
        return "Student{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", gender='" + gender + '\'' +
                ", age=" + age +
                ", clazz=" + clazz.getName() +
                '}';
    }
}

（三）创建班级实体类

package net.tjl.mybatis.bean;
/**
 * 功能：
 * 作者：谭金兰
 * 日期：2021年03月10日
 */

import java.util.List;

public class Clazz {
    
    
    private int id;
    private String name;
    private Teacher teacher;
    private List<Student> students;

    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;
    }

    public Teacher getTeacher() {
    
    
        return teacher;
    }

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

    public List<Student> getStudents() {
    
    
        return students;
    }

    public void setStudents(List<Student> students) {
    
    
        this.students = students;
    }

    @Override
    public String toString() {
    
    
        return "Clazz{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", teacher=" + teacher.getName() +
                ", students=" + students +
                '}';
    }
}

三、创建班级映射器配置文件

<?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="net.tjl.mybatis.mapper.ClazzMapper">
    <!--三表关联查询-->
    <select id="findById" parameterType="int" resultMap="clazzResultMap">
        SELECT * FROM t_teacher INNER JOIN t_class ON t_class.teacher_id = t_teacher.t_id
        INNER JOIN t_student ON t_class.c_id = t_student.class_id WHERE c_id = #{
    
    id};
    </select>

    <!--定义班级结果映射-->
    <resultMap id="clazzResultMap" type="Clazz">
        <result property="id" column="c_id"/>
        <result property="name" column="c_name"/>
        <!--一对一-->
        <association property="teacher" column="teacher_id" javaType="Teacher">
            <result property="id" column="t_id"/>
            <result property="name" column="t_name"/>
        </association>
        <!--一对多-->
        <collection property="students" ofType="Student">
            <result property="id" column="s_id"/>
            <result property="name" column="s_name"/>
            <result property="gender" column="s_gender"/>
            <result property="age" column="s_age"/>
            <association property="clazz" column="class_id" javaType="Clazz">
                <result property="name" column="c_name"/>
            </association>
        </collection>
    </resultMap>



    <!--通过嵌套查询实现一对一关联-->
    <select id="findAll" resultMap="clazzResultMap2">
        SELECT * FROM t_class;
    </select>

    <!--定义班级结果映射-->
    <resultMap id="clazzResultMap2" type="Clazz">
        <result property="id" column="c_id"/>
        <result property="name" column="c_name"/>
        <association property="teacher" column="teacher_id" javaType="Teacher" select="getTeacher"/>
    </resultMap>

    <select id="getTeacher" resultType="Teacher">
       SELECT t_id id, t_name name FROM t_teacher WHERE t_id = #{
    
    id};
    </select>
</mapper>

四、修改MyBatis配置文件
在配置里配置班级、教师与学生实体类别名，以及班级映射器配置文件


五、定义班级映射接口ClazzMapper

package net.tjl.mybatis.mapper;


import net.tjl.mybatis.bean.Clazz;

import java.util.List;

public interface ClazzMapper {
    
    
    Clazz findById(int id);
    List<Clazz>findAll();
}

六、创建测试类TestClazzMapper

package net.tjl.mapper;

import net.tjl.mybatis.bean.Clazz;
import net.tjl.mybatis.bean.Student;
import net.tjl.mybatis.mapper.ClazzMapper;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;

import java.io.IOException;
import java.io.Reader;
import java.util.List;

/**
 * 功能：测试班级映射器接口
 * 作者：谭金兰
 * 日期：2021年03月10日
 */
public class TestClazzMapper {
    
    

    private SqlSession sqlSession; // SQL会话
    private ClazzMapper clazzMapper; // 班级映射器

    @Before
    public void init() {
    
    
        try {
    
    
            // 读取MyBatis配置文件作为字符输入流
            Reader reader = Resources.getResourceAsReader("mybatis-config.xml");
            // 基于MyBatis配置文件构建SQL会话工厂
            SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(reader);
            // 利用SQL会话工厂获取SQL会话
            sqlSession = factory.openSession();
            // 利用SQL会话获取班级映射器对象
            clazzMapper = sqlSession.getMapper(ClazzMapper.class);
            System.out.println("sqlSession对象已创建");
        } catch (IOException e) {
    
    
            e.printStackTrace();
        }
    }

    @Test
    public void testFindById() {
    
    
        int id = 1;
        Clazz clazz = clazzMapper.findById(id);
        System.out.println(clazz);

        System.out.println("班级编号[" + id + "]的学生：");
        List<Student> students = clazz.getStudents();
        students.forEach(student -> System.out.println(student));
    }

    @Test
    public void testFindAll() {
    
    
        List<Clazz> clazzes = clazzMapper.findAll();
        clazzes.forEach(clazz -> System.out.println(clazz));
    }

    @After
    public void destroy() {
    
    
        // 关闭SQL会话
        sqlSession.close();
        //提示用户SQL会话对象关闭
        System.out.println("sqlSession对象已关闭");
    }
}

（一）运行测试方法testFindById()，查看结果

（二）运行测试方法testFindAll()，查看结果