Mybatis关联映射举例详解

public class Student {
    private Integer id;
    private String Sname;
    private String sex;
    private Integer age;
    private Integer t_id;
    //这个是重点
    private Teacher teacher;
}

public class Teacher {
    private Integer id;
    private String Tname;
}

SELECT  student.id,student.name,teacher.name FROM student LEFT JOIN teacher  on student.t_id = teacher.id

   <!--    按照结果嵌套处理-->
<select id="getStudent1" resultMap="StudentTeacher1">
   SELECT  student.id,student.Sname,teacher.Tname FROM student  LEFT JOIN teacher  on student.t_id = teacher.id
</select>
   <resultMap id="StudentTeacher1" type="com.qcby.entity.Student">
       <result property="id" column="id"/>
       <result property="Sname" column="Sname"/>
       <result property="sex" column="sex"/>
       <result property="age" column="age"/>
       <result property="t_id" column="t_id"/>
     <!-- 复杂的属性我们需要单独去处理 对象:association   集合：collection   -->
   	 <!-- property="teacher" student类当中的关联字段 -->
   	 <!-- javaType="com.javen.model.Teacher" 为复杂属性设置类类型-->	
       <association property="teacher" javaType="com.qcby.entity.Teacher">
           <result property="id" column="id"/>
           <result property="Tname" column="Tname"/>
       </association>
   </resultMap>

SELECT s.id,s.Sname,t.Tname FROM student s,teacher t where s.t_id = t.id

<select id = "getStudent"  resultMap="StudentTeacher">
    select * from student;
</select>
<!--结果映射集-->
<resultMap id="StudentTeacher"  type="com.qcby.entity.Student">
    <result property="id" column="id"/>
    <result property="Sname" column="Sname"/>
    <result property="sex" column="sex"/>
    <result property="age" column="age"/>
    <result property="t_id" column="t_id"/>
    <!-- select="getTeacher"  :调用下一个查询语句       -->
    <!-- column="t_id" 两个表的关联字段-->
    <association property="teacher" column="t_id" javaType="com.qcby.entity.Teacher" select="getTeacher"/>
</resultMap>
<select id="getTeacher" resultType="com.qcby.entity.Teacher">
    select  * from  teacher where id = #{t_id};    <!-- #{id}; 可以写任何东西，因为会自动匹配 t_id -->
</select>

public class Teacher {
    private Integer id;
    private String Tname;
    //这个一定要有
    private List<Student> students;
}

<!--按照结果进行查询-->
<select id="getTeacher" resultMap="TeacherStudent">
    SELECT  teacher.id,teacher.Tname,student.Sname FROM teacher
        LEFT JOIN student  on student.t_id = teacher.id
 </select>
<resultMap id="TeacherStudent" type="com.qcby.entity.Teacher">
    <result property="id" column="id"/>
    <result property="Tname" column="Tname"/>
    <!-- 复杂的属性我么需要单独去处理 对象:association   集合：collection
      在集合中的泛型信息，我们使用ofType获取
      -->
    <collection property="students" ofType="com.qcby.entity.Student">
    	<!-- 查询什么写什么 -->
        <result property="Sname" column="Sname"/>
    </collection>
</resultMap>

<!--按照查询嵌套处理：分布查询-->
<select id="getTeacher" resultMap="TeacherStudent2">
    select * from teacher
</select>
<resultMap id="TeacherStudent2" type="com.qcby.entity.Teacher">
    	<!--column="t_id"  传值-->
    <collection property="students" column="t_id"  
                ofType="com.qcby.entity.Student" select="getStudentByTeacherId" />  <!--实现分布查询-->
</resultMap>
<select id="getStudentByTeacherId" resultType="com.qcby.entity.Student">
    select * from student where id = #{t_id}
</select