MyBatis框架-多对一处理

多对一的处理

• 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, '王老师'); 
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');

实体类

package com.sin.pojo;

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

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

package com.sin.pojo;

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

@Data
@AllArgsConstructor
@NoArgsConstructor
@ToString
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;
}

Mapper层

public interface StudentMapper {}

public interface TeacherMapper {}

StudentMapper.xml

DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.sin.mapper.StudentMapper">
mapper>

TeacherMapper.xml

DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.sin.mapper.TeacherMapper">
mapper>

StudentMapper.java

//获取所有学生及对应老师的信息
public List<Student> getStudents();

StudentMapper.xml

DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.sin.mapper.StudentMapper">

    
    <resultMap id="StudentTeacher" type="com.sin.pojo.Student">
        
        
        <association property="teacher"  column="tid" javaType="com.sin.pojo.Teacher" select="getTeacher"/>
    resultMap>
    
    <select id="getStudents" resultMap="StudentTeacher">
      select * from student
    select>
    
    <select id="getTeacher" resultType="com.sin.pojo.Teacher">
        select * from teacher where id = #{tid}
    select>
mapper>

Mybatis-config.xml

注册Mapper

<mappers>
    <mapper resource="mapper/StudentMapper.xml"/>
mappers>

测试

  @Test
    public void testGetStudents(){
        SqlSession session = MyBatisUtil.getSession();
        StudentMapper mapper = session.getMapper(StudentMapper.class);
        List<Student> students = mapper.getStudents();
        for (Student student: students){
            System.out.println("学生名:"+ student.getName()+",老师:"+student.getTeacher().getName());
        }
    }

测试结果

关联查询

上面的方法，是查询完学生，再一个一个查询老师，感觉性能会有点慢。

除了上面这种方式，还可以多表查询，一次查询出来。

StudentMapper.java

public List<Student> getStudents2();

StudentMapper.xml

 
    <resultMap id="StudentTeacher2" type="com.sin.pojo.Student">
        <id property="id" column="sid"/>
        <result property="name" column="sname"/>
        
        <association property="teacher" javaType="com.sin.pojo.Teacher">
            
            <result property="name" column="tname"/>
        association>
    resultMap>
    
    <select id="getStudents2" resultMap="StudentTeacher2" >
    select s.id sid, s.name sname , t.name tname
    from student s,teacher t
    where s.tid = t.id
select>

测试

@Test
public void testGetStudents2(){
    SqlSession session = MyBatisUtil.getSession();
    StudentMapper mapper = session.getMapper(StudentMapper.class);
    List<Student> students = mapper.getStudents2();
    for (Student student : students){
        System.out.println("学生名:"+ student.getName()+",老师:"+student.getTeacher().getName());
    }
}

测试结果

总结

• 按照查询进行嵌套处理就像SQL中的子查询（类似for里面再调用方法）
• 按照结果进行嵌套处理就像SQL中的联表查询（通过分组来封装结果）