Mybatis多对一的处理

多对一关系
下面设计了两张表，根据学生来查询

表的设计
利用Navicat for mysql建表，然后在查看菜单点击ER图表（概念模型） 如下：

设计好的表ER图表

上述表 student tid设置外键参考teacher的id，所以建好表后两者都为空要先添加teacher的数据。

---

多个学生对于一个老师
实体类
Teacher

public class Teacher {
    private int id;
    private String name;
    //省略set/get方法
    @Override
    public String toString() {
        return "Teacher [id=" + id + ", name=" + name + "]";
    }
}

Student

public class Student {
    private int id;
    private String name;
    private Teacher teacher;
    //省略set/get方法
    @Override
    public String toString() {
        return "Student [id=" + id + ", name=" + name + ", teacher=" + teacher
                + "]";
    }
}

编写映射文件

按结果嵌套处理

<mapper namespace="cn.sxt.entity.student.mapper">
    <!--  多对一处理有两种方式
            1 .按结果嵌套处理
            2 .按查询嵌套处理
    -->
    <!--按结果嵌套处理  -->
    <select id="getStudents"  resultMap="StudentTeacher">
        select  s.id sid,s.name sname,s.tid stid ,t.id tid,t.name tname from student s,teacher t where s.tid=t.id
    </select>
    <resultMap type="Student" id="StudentTeacher">
        <id column="sid"  property ="id"></id>
        <result column="sname"  property="name"></result>
        <!-- 关联对象    property  关联对象在Student实体类的属性  javaType  property对于属性的类型-->
        <association property="teacher"  javaType="Teacher">
            <id  column="tid"  property="id"></id>
            <result  column="tname" property="name"></result>
        </association>  
    </resultMap>
</mapper>

按查询结果嵌套处理

<!--按查询嵌套处理  -->
     <select id="getStudents"  resultMap="StudentTeacher">
        select * from  student
     </select>
     <resultMap type="Student" id="StudentTeacher">
         <association property="teacher" column="tid"  javaType="Teacher" select="cn.sxt.entity.teacher.mapper.getTeacher"></association>
     </resultMap>

teacher.mapper.xml中

<mapper namespace="cn.sxt.entity.teacher.mapper">
    <select id="getTeacher" resultType="Teacher">
    select * from teacher where id=#{id}
    </select>
</mapper>

测试

public static void main(String[] args) throws IOException {
        StudentDao studentDao=new StudentDao();
        List<Student> list=studentDao.getAll();
        for(Student stu:list){
            System.out.println("student name"+stu.getName()+"  student's teacher name="+stu.getTeacher().getName());
        } 
    }