MyBatis中多对一

多对一的处理

多对一的理解：

• 多个学生对应一个老师
• 如果对于学生这边，就是一个多对一的现象，即从学生这边关联一个老师！

数据库设计

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, '程老师'); 

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'); 

实例：

下面的例子是在SpringBoot环境下搭建的。这里有两种方式来进行多对一的查询。

• 按照查询进行嵌套处理就像SQL中的子查询
• 按照结果进行嵌套处理就像SQL中的联表查询

只举一种来说，按照结果进行嵌套处理就像SQL中的联表查询。比较好理解。

1. pom.xml 依赖

        <dependency>
            <groupId>org.mybatis.spring.boot</groupId>
            <artifactId>mybatis-spring-boot-starter</artifactId>
            <version>2.1.1</version>
        </dependency>
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
        </dependency>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <scope>test</scope>
        </dependency>

2. application.properties

mybatis.mapper-locations=classpath:mapper/*.xml
mybatis.type-aliases-package=com.cc.pojo
mybatis.config-location=classpath:mybatis-config.xml

spring.datasource.username=root
spring.datasource.password=123456
spring.datasource.url=jdbc:mysql://localhost:3306/mis?serverTimezone=Asia/Shanghai&useUnicode=true&characterEncoding=utf-8

3.日志配置（这里采用mybatis标准的日志。STDOUT_LOGGING。）

mybatis-config.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
    <settings>
        <setting name="logImpl" value="STDOUT_LOGGING" />
    </settings>
</configuration>

4 主要代码

1） pojo包下的Student类，Teacher类。

package com.cc.pojo;


public class Student {
    private int id;
    private String name;
    //多个学生可以是同一个老师，即多对一
    private Teacher teacher;

    //GET,SET,ToString，有参，无参构造
}

package com.cc.pojo;

public class Teacher {
    private int id;
    private String name;
    //GET,SET,ToString，有参，无参构造
}

2）mapper下的接口。

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package com.cc.mapper;

import com.cc.pojo.Student;
import org.apache.ibatis.annotations.Mapper;
import java.util.List;

@Mapper
public interface StudentMapper {
    //获取所有学生及对应老师的信息
    public List<Student> getStudents();
}

@Mapper
public interface TeacherMapper {

}

这里TeacherMapper虽然没有方法，但写上以备后续不时之需。

3）编写Mapper接口对应的 mapper.xml配置文件 

StudentMapper.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.cc.mapper.StudentMapper">
    <!--
    按查询结果嵌套处理
    思路：
        1. 直接查询出结果，进行结果集的映射
    -->
    <select id="getStudents" resultMap="StudentTeacher" >
        select s.id sid, s.name sname , t.name tname
        from student s,teacher t
        where s.tid = t.id
    </select>

    <resultMap id="StudentTeacher" type="Student">
        <id property="id" column="sid"/>
        <result property="name" column="sname"/>
        <!--关联对象property 关联对象在Student实体类中的属性-->
        <association property="teacher" javaType="Teacher">
            <result property="name" column="tname"/>
        </association>
    </resultMap>
</mapper>

TeacherMapper.xml 

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.cc.mapper.TeacherMapper">

</mapper>

4）测试

package com.cc;

import com.cc.mapper.StudentMapper;
import com.cc.pojo.Student;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.context.junit4.SpringRunner;

import java.util.List;

@RunWith(SpringRunner.class)
@SpringBootTest
public class StudentTest {

    @Autowired
    private StudentMapper studentMapper;

    @org.junit.Test
    public void testStudent() {
        List<Student> students = studentMapper.getStudents();

//        System.out.println(students.toString());
        for (Student student : students){
            System.out.println(
                    "学生名:"+ student.getName()
                            +"\t老师:"+student.getTeacher().getName());
        }
    }
}

5）运行结果