Mybatis实现关联表查询（一对多关联）

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 teacher(

t_id INT PRIMARY KEY AUTO_INCREMENT,

t_name VARCHAR(20)

);

CREATE TABLE class(

c_id INT PRIMARY KEY AUTO_INCREMENT,

c_name VARCHAR(20),

teacher_id INT

);

ALTER TABLE class ADD CONSTRAINT fk_teacher_id FOREIGN
KEY (teacher_id) REFERENCES

teacher(t_id);

INSERT INTO teacher(t_name) VALUES('LS1');

INSERT INTO teacher(t_name) VALUES('LS2');

INSERT INTO class(c_name, teacher_id) VALUES('bj_a', 1);

INSERT INTO class(c_name, teacher_id) VALUES('bj_b', 2);

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 Teacher {

private int id;

private String name;

}

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>//collection做一对多关联的，ofType指定集合中元素对象类型

</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>

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);

}