十关联映射

Posted 2022-03-29 &小小白&

十、关联映射

关联映射指如何将多个有关联的表映射到对应的关联对象中，对象之间的关联关系是有方向的可以是单向也可以是双向。

10.1、创建表

创建老师表与学生表

CREATE TABLE `teacher`  (
  `t_id` int(10) NOT NULL,
  `t_name` varchar(30) DEFAULT NULL,
  PRIMARY KEY (`t_id`)
) ENGINE = InnoDB DEFAULT CHARSET = utf8

INSERT INTO `teacher` (`t_id`,`t_name`) VALUES (1, '李老师');

CREATE TABLE `student`  (
  `s_id` int(10) NOT NULL,
  `s_name` varchar(30) DEFAULT NULL,
  `tea_id` int(10) DEFAULT NULL,
  PRIMARY KEY (`s_id`),
  KEY `fktid`(`tea_id`),
  CONSTRAINT `fktid` FOREIGN KEY (`tea_id`) REFERENCES `teacher` (`t_id`)
) ENGINE = InnoDB DEFAULT CHARSET = utf8

INSERT INTO `student` (`s_id`,`s_name`,`tea_id`) VALUES (1, '小明', 1);
INSERT INTO `student` (`s_id`,`s_name`,`tea_id`) VALUES (2, '小红', 1);
INSERT INTO `student` (`s_id`,`s_name`,`tea_id`) VALUES (3, '小张', 1);
INSERT INTO `student` (`s_id`,`s_name`,`tea_id`) VALUES (4, '小李', 1);
INSERT INTO `student` (`s_id`,`s_name`,`tea_id`) VALUES (5, '小王', 1);

10.2、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我们的Mapper接口或者文件

6. 测试查询是否能够成功

10.3、一对多关联

处理的问题：获取指定老师下面的所有学生及老师的信息

<select id="getTeacher" resultMap="TS">
    select t.t_id,t.t_name,s.s_id,s.s_name from student s,teacher t where s.tea_id = t.t_id and t.t_id=#tea_id
</select>
<resultMap id="TS" type="Teacher">
    <result property="t_id" column="t_id"/>
    <result property="t_name" column="t_name"/>
    <collection property="students" ofType="Student">
    	<result property="s_id" column="s_id"/>
    	<result property="s_name" column="s_name"/>
    	<result property="tea_id" column="tea_id"/>
    </collection>
</resultMap>

10.4、多对一关联

处理的问题：查询所有的学生信息，以及对应的老师的信息。

10.4.1、按照查询嵌套处理

<select id="getStudent" resultMap="StudentTeacher">
    select * from student
</select>

<resultMap id="StudentTeacher" type="Student">
    <result property="s_id" column="s_id"/>
    <result property="s_name" column="s_name"/>
    <!--association:对象   collectionn:集合-->
    <association property="teacher" column="tea_id" javaType="Teacher" select="getTeacher"/>
</resultMap>

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

10.4.2、按照结果嵌套处理

<select id="getStudent2" resultMap="StudentTeacher2">
    select s.s_id,s.s_name,t.t_name from student s,teacher t where s.tea_id = t.t_id
</select>

<resultMap id="StudentTeacher2" type="Student">
    <result property="s_id" column="s_id"/>
    <result property="s_name" column="s_name"/>
    <!--association:对象   collectionn:集合-->
    <association property="teacher" javaType="Teacher">
    	<result property="t_name" column="t_name"/>
    </association>
</resultMap>