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);

SELECT * FROM class c,teacher t WHERE c.teacher_id=t.t_id AND c.c_id=1

select * from class where c_id=1
select * from teacher where t_id=1

public class Classes {
    private int id;
    private String name;
    private Teacher teacher;
}

..省略了get set方法，及默认构造方法

public class Teacher {
    private int id;
    private String name;
}
省略了get set方法，及空构造方法

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="zxj.domain.ClassMapper">
    <!-- 根据id来查询班级信息（带老师的信息） -->
    <!-- 查询方法sql:SELECT * FROM class c,teacher t WHERE c.teacher_id=t.t_id AND c.c_id=1 -->
    <select id="getClass" parameterType="int" resultMap="getClassMap">
        SELECT * FROM class c,teacher t WHERE c.teacher_id=t.t_id AND c.c_id=#{id}
    </select>
    <!-- 在返回的结果集中，指定每一个列，对应的是哪一个对象，使用association标签来声明其它的对象，即一对一 -->
    <resultMap type="zxj.domain.Classes" id="getClassMap">
        <id property="id" column="c_id"/>
        <result column="c_name" property="name"/>
        <association property="teacher" javaType="zxj.domain.Teacher">
            <id property="id" column="t_id"/>
            <result property="name" column="t_name"/>
        </association>
    </resultMap>
</mapper>

/**
     * 测试一对一的查询<br>
     * 查询sql:SELECT * FROM class c,teacher t WHERE c.teacher_id=t.t_id AND c.c_id=1
     */
    @Test
    public void testOneToOne() {
        InputStream in = TestMybaits.class.getClassLoader().getResourceAsStream("conf.xml");
        SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(in);
        SqlSession session = factory.openSession();
        String statement = "zxj.domain.ClassMapper.getClass";
        Classes c = session.selectOne(statement, 1);
        System.out.println(c);
        session.close();
    }

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="zxj.domain.ClassMapper">

    
    <!-- 另一种查询一对一的方法 -->
    <!-- 
        方法二：select * from class where c_id=1
                select * from teacher where t_id=1(这里的t_id是上一条sql得到的id)
    -->
    <select id="getClass2" parameterType="int" resultMap="getClassMap2">
        select * from class where c_id=#{id}
    </select>
    
    <select id="getTeacher" parameterType="int" resultType="zxj.domain.Teacher">
        <!-- 这里需要取别名，因为数据库中的字段，与实体类中的字段不一至，我们需要让它一至，才能封装成对象 -->
        select t_id id,t_name name from teacher where t_id=#{id}
    </select>
    <resultMap type="zxj.domain.Classes" id="getClassMap2">
        <id column="c_id" property="id"/>
        <result column="c_name" property="name"/>
        <!-- 实现原理是查询两次sql，第二次查询的条件是根据第一次查询的结果，下面的column属性就是要传过去查询的参数 -->
        <association property="teacher" column="teacher_id" select="getTeacher" >
        </association>
    </resultMap>
</mapper>

    /**
     * 方式二<br>
     * 测试一对一的查询<br>
     * 查询sql:SELECT * FROM class c,teacher t WHERE c.teacher_id=t.t_id AND c.c_id=1
     */
    @Test
    public void testOneToOne2() {
        InputStream in = TestMybaits.class.getClassLoader().getResourceAsStream("conf.xml");
        SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(in);
        SqlSession session = factory.openSession();
        String statement = "zxj.domain.ClassMapper.getClass2";
        Classes c = session.selectOne(statement, 1);
        System.out.println(c);
        session.close();
    }