MyBatis笔记06 - 一对多关联查询

6.2. 一对多关联查询

1). 提出需求
根据classId查询对应的班级信息,包括学生,老师

2). 创建表和数据：

CREATE TABLE student(
s_id INT PRIMARY KEY AUTO_INCREMENT,
s_name VARCHAR(20),
class_id INT
);
INSERT INTO student(s_name, class_id) VALUES('xs_A', 1);
INSERT INTO student(s_name, class_id) VALUES('xs_B', 1);
INSERT INTO student(s_name, class_id) VALUES('xs_C', 1);
INSERT INTO student(s_name, class_id) VALUES('xs_D', 2);
INSERT INTO student(s_name, class_id) VALUES('xs_E', 2);
INSERT INTO student(s_name, class_id) VALUES('xs_F', 2);

 

3). 定义实体类：

public class Student {
private int id;
private String name;
}

public class Classes {
private int id;
private String name;
private Teacher teacher;
private List<Student> students;
}

4). 定义sql映射文件ClassMapper.xml

<!--
方式一: 嵌套结果: 使用嵌套结果映射来处理重复的联合结果的子集
SELECT * FROM class c, teacher t,student s WHERE c.teacher_id=t.t_id AND c.C_id=s.class_id AND  c.c_id=1
-->
<select id="getClass3" parameterType="int" resultMap="ClassResultMap3">
select * from class c, teacher t,student s where c.teacher_id=t.t_id and c.C_id=s.class_id and  c.c_id=#{id}
</select>
<resultMap type="_Classes" id="ClassResultMap3">
<id property="id" column="c_id"/>
<result property="name" column="c_name"/>
<association property="teacher" column="teacher_id" javaType="_Teacher">
<id property="id" column="t_id"/>
<result property="name" column="t_name"/>
</association>
<!-- ofType指定students集合中的对象类型 -->
<collection property="students" ofType="_Student">
<id property="id" column="s_id"/>
<result property="name" column="s_name"/>
</collection>
</resultMap>

<!--
方式二：嵌套查询：通过执行另外一个SQL映射语句来返回预期的复杂类型
SELECT * FROM class WHERE c_id=1;
SELECT * FROM teacher WHERE t_id=1   //1 是上一个查询得到的teacher_id的值
SELECT * FROM student WHERE class_id=1  //1是第一个查询得到的c_id字段的值
-->
<select id="getClass4" parameterType="int" resultMap="ClassResultMap4">
select * from class where c_id=#{id}
</select>
<resultMap type="_Classes" id="ClassResultMap4">
<id property="id" column="c_id"/>
<result property="name" column="c_name"/>
<association property="teacher" column="teacher_id" javaType="_Teacher" select="getTeacher2"></association>
<collection property="students" ofType="_Student" column="c_id" select="getStudent"></collection>
</resultMap>

<select id="getTeacher2" parameterType="int" resultType="_Teacher">
SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id}
</select>

<select id="getStudent" parameterType="int" resultType="_Student">
SELECT s_id id, s_name name FROM student WHERE class_id=#{id}
</select>

注意：

collection : 做一对多关联的 

ofType : 指定集合中元素对象的类型 

5). 测试：

@Test
public void testOM() {
SqlSession sqlSession = factory.openSession();
Classes c = sqlSession.selectOne("com.atguigu.day03_mybatis.test5.OOMapper.getClass3", 1);
System.out.println(c);
}

@Test
public void testOM2() {
SqlSession sqlSession = factory.openSession();
Classes c = sqlSession.selectOne("com.atguigu.day03_mybatis.test5.OOMapper.getClass4", 1);
System.out.println(c);
}