05_一对一关联表查询：有二种方式

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
);
//在班级表中加一个外键，指向老师表的主键
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;
}

<!-- 方式一：嵌套结果 -->
	<resultMap type="Classes" id="ClassResultMap">
		<id property="id" column="c_id"/>
		<result property="name" column="c_name"/>
		<association property="teacher" javaType="Teacher">
			<id property="id" column="t_id"/>
			<result property="name" column="t_name"/>
		</association>
	</resultMap>
	<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>

• 测试
  

@Test
	public void test1(){
		SqlSessionFactory factory = MybatisUtils.getFactory();
		SqlSession session = factory.openSession();
		String statement = "cn.imentors.mybatis.test5.classMapper.getClass";
		Classes c = session.selectOne(statement , 2);
		System.out.println(c);
		session.close();
	}

<!-- 方式：嵌套查询：本质是二次查询 -->
	<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"/>
		<!-- column:是classes表中外键字段 -->
		<association property="teacher" column="teacher_id" 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>

	@Test
	public void test2(){
		SqlSessionFactory factory = MybatisUtils.getFactory();
		SqlSession session = factory.openSession();
		String statement = "cn.imentors.mybatis.test5.classMapper.getClass2";
		Classes c = session.selectOne(statement , 2);
		System.out.println(c);
		session.close();
	}