文章目录

• 1. 测试环境搭建
• 2. 实例测试

本文章涉及环境版本：

• mysql 5.7
• Mybatis 3.5.x
• Maven 3.6.x
• JDK 1.8

项目代码仓库：
https://github.com/Gang-bb/Study-Record/tree/main/bzhan-mybatis-study
需要clone整个bzhan-mybatis-study项目
(整体是一个maven多module工程)
该文章对应：《bzhan-mybatis-study06 》module项目

1. 测试环境搭建

主要关键类代码修改：

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

@Data
@NoArgsConstructor
@AllArgsConstructor
public class Teacher {
    private Integer id;
    private String name;
    
    private List<Student> students;
}

2. 实例测试

查询教师及其所有学生

方式一：按照结果嵌套处理

1. TeacherMapper

Teacher getTeacher();

2. TeacherMapper.xml

<!--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，我们需要单独处理 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>

3. 测试类

@Test
public void getTeacher(){
    SqlSession sqlSession = null;
    try {
        sqlSession = MybatisUtil.openSession();
        TeacherMapper teacherMapper = sqlSession.getMapper(TeacherMapper.class);

        System.out.println(teacherMapper.getTeacher(1));
    } finally {
        sqlSession.close();
    }
}

4. 结果

方式二：按照查询嵌套处理

1. TeacherMapper

Teacher getTeacher2();

2. TeacherMapper.xml

<!-- 一对多模型 按照查询嵌套处理-->
<select id="getTeacher2" resultMap="TeacherStudent2">
    select id, name from teacher where id = #{id}
</select>

<resultMap id="TeacherStudent2" type="Teacher">
        <result property="id" column="id"/>
        <result property="name" column="name"/>
    <collection property="students" javaType="ArrayList" ofType="Student" select="getStudentByTeacherId" column="id"/>
</resultMap>

<select id="getStudentByTeacherId" resultType="Student">
    select * from student where tid = #{tid}
</select>

3. 测试类

@Test
public void getTeacher(){
    SqlSession sqlSession = null;
    try {
        sqlSession = MybatisUtil.openSession();
        TeacherMapper teacherMapper = sqlSession.getMapper(TeacherMapper.class);

        System.out.println(teacherMapper.getTeacher2(1));
    } finally {
        sqlSession.close();
    }
}

4. 结果

注意点：

• 保证SQL的可读性，尽量保证通俗易懂
• 注意一对多和多对一中，属性名和字段的问题！