06_一对多联表查询

1 需求

• 一个班级中有多个学生
• 根据classId查询对应的班级信息，包括学生信息和老师信息

2 创建表和数据

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

• Student

public class Student {
	private int id;
	private String name;
}

• 修改Classes类，添加一个学生集合属性

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

4.1 方式一：嵌套结果，也就是连表查询

• xml

<!-- 方式一：嵌套结果 ,连表查询
	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
	-->
	<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>
	<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>

• 测试

@Test
	public void testOM() {
		SqlSessionFactory factory = MybatisUtils.getFactory();
		SqlSession sqlSession = factory.openSession();
		Classes c = sqlSession.selectOne("cn.imentors.mybatis.test6.classMapper.getClass3", 1);
		System.out.println(c);
	}

4.2 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

• xml

<!-- 
	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>
 </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>

测试

@Test
	public void testOM2() {
		SqlSessionFactory factory = MybatisUtils.getFactory();
		SqlSession sqlSession = factory.openSession();
		Classes c = sqlSession.selectOne("cn.imentors.mybatis.test6.classMapper.getClass4", 1);
		System.out.println(c);
	}

5 属性说明

• collection:做一对多关联查询
• ofType:指定集合中元素对象的类型

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