Mybatis实现关联表查询（一对多关联）

根据classId 查询对应的班级信息,包括学生,老师

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);

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：

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

<!-- 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 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 --> <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>

<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> <!-- 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>//collection做一对多关联的，ofType指定集合中元素对象类型 </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>

5). 测试： @Test public void testOM() { SqlSession sqlSession = factory.openSession(); Classes c = sqlSession.selectOne("com.atguigu.day03_mybatis.test5.OOMapper.getClass3", 1); System.out.println(c); } @Test public void testOM2() { SqlSession sqlSession = factory.openSession(); Classes c = sqlSession.selectOne("com.atguigu.day03_mybatis.test5.OOMapper.getClass4", 1); System.out.println(c); }