Mybatis【多对一处理】

多对一处理

多个学生，对应一个老师

对于学生这边而言，关联...多个学生，关联一个老师【多对一】

对于老师而言，集合，一个老师有很多学生【一对多】

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, '秦老师'); 

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

 <!--lombok-->
        <dependency>
                <groupId>org.projectlombok</groupId>
                <artifactId>lombok</artifactId>
                <version>1.18.22</version>
                <scope>compile</scope>
        </dependency>

2.新建实体类Teacher,Student

package com.jiang.pojo;


import lombok.Data;

@Data
public class Student {
    private int id;
    private String name;

    //学生需要关联一个老师
    private Teacher teacher;
}

package com.jiang.pojo;

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

@Data
@AllArgsConstructor
@NoArgsConstructor
public class Teacher {
    private int id;
    private String name;
}

3.建立Mapper接口

package com.jiang.mapper;

import com.jiang.pojo.Teacher;
import org.apache.ibatis.annotations.Param;
import org.apache.ibatis.annotations.Select;

public interface TeacherMapper {

    @Select("select * from teacher where id=#{tid}")
    Teacher getTeacher(@Param("tid") int id);


}

package com.jiang.mapper;

public interface StudentMapper {
}

4.建立Mapper.XML文件

<?xml version="1.0" encoding="UTF8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

<mapper namespace="com.jiang.mapper.TeacherMapper">

</mapper>

<?xml version="1.0" encoding="UTF8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

<mapper namespace="com.jiang.mapper.StudentMapper">

</mapper>

 5.在核心文件中绑定注册我们的Mapper接口或者文件！

<mappers>
    <mapper class="com.jiang.mapper.TeacherMapper"></mapper>
    <mapper class="com.jiang.mapper.StudentMapper"></mapper>
</mappers>

6.测试查询是否能够成功

import com.jiang.mapper.TeacherMapper;
import com.jiang.pojo.Teacher;
import com.jiang.util.MybatisUtils;
import org.apache.ibatis.session.SqlSession;

public class Mytest {
    public static void main(String[] args) {
        SqlSession sqlSesssion = MybatisUtils.getSqlSesssion();
        TeacherMapper mapper = sqlSesssion.getMapper(TeacherMapper.class);
        Teacher teacher = mapper.getTeacher(1);
        System.out.println(teacher);
        sqlSesssion.close();

    }
}

按照查询嵌套处理

 <!--
    思路
        1.查询所有的学生信息
        2.根据查询出来学生的tid，寻找对应的老师
    -->
    <select id="getStudent" resultMap="StudentTeacher">
        select * from student
    </select>
    <resultMap id="StudentTeacher" type="Student">
        <result property="id" column="id"/>
        <result property="name" column="name"/>
        <!--复杂的属性，我们需要单独处理
            对象：association
            集合：collection
        -->
        <association property="teacher" column="tid" javaType="Teacher" select="getTeacher"/>
    </resultMap>

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

按照结果嵌套处理

<!--按照结果嵌套处理-->
    <select id="getStudent2" resultMap="StudentTeacher2">
        select s.id sid,s.name sname,t.name tname
        from student s ,teacher t
        where s.tid=t.id
    </select>

    <resultMap id="StudentTeacher2" type="Student">
        <result property="id" column="sid"/>
        <result property="name" column="sname"/>
        <association property="teacher" javaType="Teacher">
            <result property="name" column="tname"/>
        </association>
    </resultMap>

回顾Mysql多对一查询方式

子查询

联表查询

https://www.bilibili.com/video/BV1NE411Q7Nx?p=21&spm_id_from=pageDriver