Mybatis框架-20：多表操作之多对多——查询所有

以你之姓@ 2022-01-21 07:29 183阅读 0赞

这里我建立的多对多的关系是教师和学生之间的关系

多表之间的查询我们仍然可以使用左连接查询

第一步：mapper

教师的mapper

/**
         * 查询老师，并把关联的学生也查出来
         * @return
         */
        List<Teacher> getAllTeacher();

第二步：mapper.xml  
由于多对多其实就是双方的一对多，本质上还是可以理解为一对多关系，所以结果的封装和一对多没有太大的区别，主要是sql的不同

<resultMap id="teacherMapper" type="com.ctbu.domain.Teacher">
            <id property="teacherId" column="teacherId"/>
            <result property="teacherName" column="teacherName"/>
            <collection property="students" javaType="list" ofType="com.ctbu.domain.Student">
                <id property="studentId" column="studentId"/>
                <result property="studentName" column="studentName"/>
            </collection>
        </resultMap>
    
        <select id="getAllTeacher" resultMap="teacherMapper">
            SELECT * FROM teacher t
            LEFT JOIN teacher_student ts on
             t.teacherId=ts.teacherId
             LEFT JOIN student s on
             ts.studentId=s.studentId;
        </select>

第三步：测试类

@Test
        public void test1(){
            SqlSession session = MyUtils.openSession();
            TeacherMapper teacherMapper = session.getMapper(TeacherMapper.class);
            List<Teacher> teachers = teacherMapper.getAllTeacher();
    
            for (Teacher teacher : teachers) {
                System.out.println(teacher);
            }
            session.close();
        }