数据库多对一查询

SQL：

CREATE TABLE `teacher` (
  `id` INT(10) NOT NULL,
  `name` VARCHAR(30) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=INNODB DEFAULT CHARSET=utf8

INSERT INTO teacher(`id`, `name`) VALUES (1, '戴老师'); 

CREATE TABLE `student` (
  `id` INT(10) NOT NULL,
  `name` VARCHAR(30) DEFAULT NULL,
  `tid` INT(10) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `fktid` (`tid`),
  CONSTRAINT `fktid` FOREIGN KEY (`tid`) REFERENCES `teacher` (`id`)
) ENGINE=INNODB DEFAULT CHARSET=utf8
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('1', '小明', '1'); 
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('2', '小红', '1'); 
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('3', '小张', '1'); 
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('4', '小李', '1'); 
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('5', '小王', '1');

测试环境搭建
1.导入lombok
2.新建实体类Teacher，Student
3.建立Mapper接口
4.建立Mapper.xml文件
5.在核心配置文件中绑定注册Mapper接口或者文件
6.测试查询是否成功

按照查询嵌套处理：

<!--    思路：
        1.查询所有学生的信息
        2.根据查询出来的学生的tid，寻找对应的老师

        -->
    <resultMap id="StudentTeacher" type="Student">
    <!-- 复杂的属性需要单独处理，对象用association， 集合用collection 
    -->
        <association property="teacher" column="tid" javaType="Teacher" select="getTeacher"
    </resultMap>
    <select id="getStudent" resultMap="StudentTeacher">
        select * from student
    </select>

    <select id="getTeacher" resultType="Teacher">
    select * from teacher
    </select>

按照结果嵌套处理

<!--    按照结果嵌套处理-->
    <select id="getStudent2" resultMap="Student2">
        select s.id  sid,s.name   sname,t.name  tname
        from student s,teacher t
        where s.tid=t.id
    </select>

    <resultMap id="Student2" type="Student">
        <result property="id" column="sid"/>
        <result property="name" column="sname"/>
        <association property="teacher" javaType="Teacher">
            <result property="name" column="tname"/>
        </association>

    </resultMap>

对应Mysql多对一查询方式：
子查询
联表查询