16.一对多处理

一个老师教多个学生

1.环境搭建

实体类

@Data
@AllArgsConstructor
@NoArgsConstructor
public class Student {
    private int id;
    private String name;
    private int tid;
}


@Data
@AllArgsConstructor
@NoArgsConstructor
public class Teacher {
    private int id;
    private String name;

    //一个老师有多个学生
    private List<Student> students;
}

2.按照查询嵌套处理

<!--1.按照查询嵌套处理-->
<select id="getTeacher2" resultMap="TeacherStudent2">
    select * from mybatis.teacher where id = #{tid}
</select>
<resultMap id="TeacherStudent2" type="Teacher">
    <collection property="students" column="id" javaType="ArrayList" ofType="Student" select="getStudentByTeacher2"/>
</resultMap>
<select id="getStudentByTeacher2" resultType="Student">
    select * from mybatis.student where tid = #{tid}
</select>

3.按照结果嵌套查询

<!--按结果嵌套查询-->
<select id="getTeacher" resultMap="TeacherStudent">
    select s.id sid,s.name sname, t.name tname,t.id tid
    from student s,teacher t
    where s.tid = t.id and t.id = #{tid};
</select>
<resultMap id="TeacherStudent" type="Teacher">
    <result property="id" column="tid"/>
    <result property="name" column="tname"/>
    
    <!--复杂的属性需要单独处理
    对象：association
    集合：collection
    javaType="" 指定属性的类型！
    集合中的泛型信息，我们使用ofType获取
    -->
    
    <collection property="students" ofType="Student">
        <result property="id" column="sid"/>
        <result property="name" column="sname"/>
        <result property="tid" column="tid"/>
    </collection>
</resultMap>