Mybatis实现关联表查询（一对一关联）

1). 提出需求

根据班级id 查询班级信息(带老师的信息)

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);

3). 定义实体类：

public class Teacher {

private int id;

private String name;

}

public class Classes {

private int id;

private String name;

private Teacher teacher;

}

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

封装联表查询的数据(去除重复的数据)

select * from class c, teacher t where c.teacher_id=t.t_id and c.c_id=1

-->

<select id="getClass" parameterType="int" resultMap="ClassResultMap">

select * from class c, teacher t where c.teacher_id=t.t_id and c.c_id=#{id}

</select>

<resultMap type="_Classes" id="ClassResultMap">

<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>

</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 id="getClass2" parameterType="int" resultMap="ClassResultMap2">

select * from class where c_id=#{id}

</select>

<resultMap type="_Classes" id="ClassResultMap2">

<id property="id" column="c_id"/>

<result property="name" column="c_name"/>

<association property="teacher" column="teacher_id" javaType="_Teacher"

select="getTeacher">

</association>

</resultMap>

<select id="getTeacher" parameterType="int" resultType="_Teacher">

SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id}

</select>

5). 测试

@Test

public void testOO() {

SqlSession sqlSession = factory.openSession();

Classes c = sqlSession.selectOne("com.atguigu.day03_mybatis.test5.OOMapper.getClass", 1);

System.out.println(c);

}

@Test

public void testOO2() {

SqlSession sqlSession = factory.openSession();

Classes c = sqlSession.selectOne("com.atguigu.day03_mybatis.test5.OOMapper.getClass2", 1);

System.out.println(c);

}