举个例子：一个老师，对应多个学生

一、复杂查询环境搭建

  1. # 创建老师表并插入数据
  2. create table `teacher` (
  3.   `id` int(10) not null,
  4.   `name` varchar(30) default null,
  5.   primary key (`id`)
  6. ) engine=innodb default charset=utf8;
  8. insert into teacher(`id`, `name`) values (1, '秦老师');
# 创建学生表并插入数据
create table `student` (
  `id` int(10) not null,
  `name` varchar(30) default null,
  `tid` int(10) default null,
  primary key (`id`),
  key `fktid` (`tid`),
  constraint `fktid` foreign key (`tid`) references `teacher` (`id`)
) engine=innodb default charset=utf8;

insert into `student` (`id`, `name`, `tid`) values 
('1', '小明', '1'),
('2', '小红', '1'),
('3', '小张', '1'),
('4', '小李', '1'),
('5', '小王', '1');

二、配置环境

//实体类，学生
public class Student {
    private int id;
    private String name;
    private int tid;

    public Student() {}

    public Student(int id, String name, int tid) {
        this.id = id;
        this.name = name;
        this.tid = tid;
    }

    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 int getTid() {
        return tid;
    }

    public void setTid(int tid) {
        this.tid = tid;
    }

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

//实体类，老师
public class Teacher {
    private int id;
    private String name;
    private List<Student> students;

    public Teacher() {}

    public Teacher(int id, String name) {
        this.id = id;
        this.name = 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;
    }

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

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

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

public interface StudentMapper {}

public interface TeacherMapper {
    //测试环境
    List<Teacher> getTeachers();
}

<?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.comprehensive.dao.TeacherMapper">

    <!-- 测试环境 -->
    <select id="getTeachers" resultType="com.comprehensive.pojo.Teacher">
        select * from mybatis.teacher
    </select>

</mapper>

public class Test_OneToMany {
    @Test
    //测试环境
    public void test() {
        SqlSession sqlSession = MyBatis_Util.getSqlSession();
        TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
        for(Teacher teacher : mapper.getTeachers()) System.out.println(teacher);
    }
}

发现问题：从结果上看，Teacher类中的private List<Student> students;属性未被赋值
image.png
解决问题：一对多实现

三、一对多

按照查询嵌套处理

  • 需求分析（SQL实现）

    select t.id as t_id, t.name as t_name from mybatis.teacher t

    public interface TeacherMapper {
  //一对多：
  //方式一：按照查询嵌套处理
  Teacher getTeacherWithStudents();
}

    ```xml <?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">


```java
public class Test_OneToMany {
    @Test
    //方式一：按照查询嵌套结果
    public void test1() {
        SqlSession sqlSession = MyBatis_Util.getSqlSession();
        TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
        Teacher teacher = mapper.getTeacherWithStudents();
        System.out.println(teacher);
        for(Student student : teacher.getStudents()) {
            System.out.println(student);
        }
        sqlSession.close();
    }
}

按照结果嵌套处理

select t.id as t_id, t.name as t_name, s.id as s_id, s.name as s_name, s.tid as s_tid
from mybatis.teacher t, mybatis.student s
where t.id=s.tid

public interface TeacherMapper {
    //一对多：
    //方式二：按照结果嵌套处理
    Teacher getTeacherWithStudents2();
}

<?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.comprehensive.dao.TeacherMapper">

    <!-- 方式二：按照结果嵌套 -->
    <select id="getTeacherWithStudents2" resultMap="teacherWithStudents2">
        select t.id as t_id, t.name as t_name, s.id as s_id, s.name as s_name, s.tid as s_tid from mybatis.teacher t, mybatis.student s where t.id=s.tid
    </select>
    <resultMap id="teacherWithStudents2" type="com.comprehensive.pojo.Teacher">
        <result property="id" column="t_id"/>
        <result property="name" column="t_name"/>
        <!-- 集合中的泛型信息，用ofType获取 -->
        <collection property="students" ofType="com.comprehensive.pojo.Student">
            <result property="id" column="s_id"/>
            <result property="name" column="s_name"/>
            <result property="tid" column="s_tid"/>
        </collection>
    </resultMap>

</mapper>

public class Test_OneToMany {
    @Test
    //方式二：按照结果嵌套
    public void test2() {
        SqlSession sqlSession = MyBatis_Util.getSqlSession();
        TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
        Teacher teacher = mapper.getTeacherWithStudents2();
        System.out.println(teacher);
        for(Student student : teacher.getStudents()) System.out.println(student);
        sqlSession.close();
    }
}