多对一

多个学生对应一个老师 图示 包结构

1；建数据库 学生：

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

老师：

CREATE TABLE `teacher` (
  `id` INT(10) NOT NULL,
  `name` VARCHAR(30) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=INNODB DEFAULT CHARSET=utf8

2：实体类 老师

public class Teacher {
          
   
    private int id;
    private String name;
}

学生（这里就需要结果集映射了）

public class Student {
          
   
    private int id;
    private String name;
    //学生需要关联一个老师
    private Teacher teacher;
}

3：实现接口(方式1) StudentMapper

public interface StudentMapper {
          
   
    List<Student> getStudent();
}

StudentMapper.xml(通过结果来进行进行查询)

<!--    ===结构集查询==-->
    <select id="getStudent2" resultMap="StudentTeacher2">
        select s.id sid,s.name sname,t.name tname
        from mybatis.student s,mybatis.teacher t
        where s.tid =t.id;
    </select>

    <resultMap id="StudentTeacher2" type="Student">
        <result property="id" column="sid"/>
        <result property="name" column="sname"/>
        <association property="teacher" javaType="Teacher">
            <result property="name" column="tname"/>
        </association>
    </resultMap>

结果：

方式2 实现接口

public interface StudentMapper {
          
   
    List<Student> getStudent2();
}

StudentMapper.xml(嵌套查询)

<!--    =========================方法二===========================-->
<!--    查询-->
    <select id="getStudent" resultMap="StudentTeacher">
        select * from mybatis.Student
    </select>

<!--    结果集映射-->
    <resultMap id="StudentTeacher" type="Student">
<!--   property="对象名" column="字段名" javaType="映射的实体类" select="嵌套的子查询"     -->
        <association property="teacher" column="tid" javaType="Teacher" select="getTeacher" />
    </resultMap>

<!--    子查询-->
    <select id="getTeacher" resultType="Teacher">
        select * from mybatis.teacher
    </select>

结果：