详解mybatis多对一关联查询的方式

private Teacher teacher;
private Classes classes;

Teacher selectTeacherById(Integer tid);

<sql id="params">tid,tname</sql>
<select id="selectTeacherById" resultType="Teacher">
    select
    <include refid="params"></include>
    from teacher where tid=#{tid}
</select>

Classes selectClassesById(Integer cid);

        <resultMap type="Classes" id="clsMap">
  <id property="cid" column="cid"></id>
  <result property="cname" column="cname"/>
 </resultMap>
 <select id="selectClassesById" resultMap="clsMap">
  select * from classes where cid = #{cid}
 </select>

Student selectStudentById(Integer sid);

        <resultMap type="Student" id="stuMap">
  <id property="sid" column="sid"/>
  <result property="sname" column="sname"/>
  <result property="age" column="age"/>
  <result property="email" column="email"/>
  <association property="teacher" select="com.yc.dao.TeacherMapper.selectTeacherById" column="tid" javaType="Teacher"></association>
  <association property="classes" select="com.yc.dao.ClassesMapper.selectClassesById" column="cid" javaType="Classes"></association>
 </resultMap>
        <select id="selectStudentById" resultMap="stuMap">
  select * from student where sid = #{sid}
 </select>

        @Test
 public void test1() {
  Student stu = studentMapper.selectStudentById(100001);
  System.out.println(stu);
 }

        <resultMap type="Student" id="stuMap">
  <id property="sid" column="sid" />
  <result property="sname" column="sname" />
  <result property="age" column="age" />
  <result property="email" column="email" />
  <association property="teacher" javaType="Teacher">
   <id property="tid" column="tid"></id>
   <result property="tname" column="tname" />
  </association>
  <association property="classes" javaType="Classes">
   <id property="cid" column="cid"></id>
   <result property="cname" column="cname" />
  </association>
 </resultMap>
 <select id="selectStudentById" resultMap="stuMap">
  select * from student s,teacher t,classes c
  where s.sid=#{sid} and s.tid = t.tid and s.cid=c.cid
 </select>

<setting name="autoMappingBehavior" value="FULL" />
<setting name="mapUnderscoreToCamelCase" value="true" />  

        <resultMap type="Student" id="stuMap">
  <association property="teacher" javaType="Teacher" />
  <association property="classes" javaType="Classes" />
 </resultMap>
 <select id="selectStudentById" resultMap="stuMap">
  select * from student s,teacher t,classes c where s.sid = #{sid} and
  s.tid=t.tid
  and s.cid=c.cid
 </select>