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);
}