Mybatis—多对一和一对多

javamagicsun

2020-02-21

关注关注

1、多对一处理

多对一：

Mybatis—多对一和一对多

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】

Mybatis—多对一和一对多

搭建环境

创建Teacher,Student表并插入

Teacher表

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');

Student表并插入

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');

导入lombok

<dependencies>
        <!-- https://mvnrepository.com/artifact/org.projectlombok/lombok -->
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <version>1.18.10</version>
        </dependency>
    </dependencies>

新建实体类 Teacher，Student

Teacher类

package com.jiang.pojo;

import lombok.Data;

import java.util.List;

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

    //一个老师拥有多个学生
    private List<Student> students;
}

Student类

package com.jiang.pojo;

import lombok.Data;

@Data
public class Student {

    private int id;
    private String name;
    private int tid;

}

建立Mapper接口
Teacher接口

//获取指定老师下的所有学生及老师的信息
    Teacher getTeacher(@Param("tid") int id);


    Teacher getTeacher2(@Param("tid") int id);

建立Mapper.XML文件
在核心配置文件中绑定注册我们的Mapper接口或者文件！
测试查询是否能够成功！

按照查询嵌套处理

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

<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>

2、一对多处理

比如：一个老师拥有多个学生！

对于老师而言，就是一对多的关系!

环境搭建

和上面一样

实体类

@Data
public class Student {

    private int id;
    private String name;
    private int tid;

}

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

    //一个老师拥有多个学生
    private List<Student> students;
}

按照结果嵌套处理

<!--按结果嵌套查询-->
    <select id="getTeacher" resultMap="TeacherStudent">
        select s.id sid, s.name sname, t.name tname,t.id tid
        from student s,teacher t
        where s.tid = t.id and t.id = #{tid}
    </select>

    <resultMap id="TeacherStudent" type="Teacher">
        <result property="id" column="tid"/>
        <result property="name" column="tname"/>
        <!--复杂的属性，我们需要单独处理 对象： association 集合： collection
        javaType="" 指定属性的类型！
        集合中的泛型信息，我们使用ofType获取
        -->
        <collection property="students" ofType="Student">
            <result property="id" column="sid"/>
            <result property="name" column="sname"/>
            <result property="tid" column="tid"/>
        </collection>
    </resultMap>

按照查询嵌套处理

<select id="getTeacher2" resultMap="TeacherStudent2">
    select * from mybatis.teacher where id = #{tid}
</select>

<resultMap id="TeacherStudent2" type="Teacher">
    <collection property="students" javaType="ArrayList" ofType="Student" select="getStudentByTeacherId" column="id"/>
</resultMap>

<select id="getStudentByTeacherId" resultType="Student">
    select * from mybatis.student where tid = #{tid}
</select>