【 MyBatis 】一對多查詢，用最簡單的例子教你如何實現一對多查詢

什麼是一對多查詢？

就是當我們查詢老師時，會同時將老師關聯的學生一同查出來，這就是一對多查詢

比如，學生表中的tid則對應老師表中的id，如果我們使用一對多查詢，查詢id爲1的老師，則學生表中tid爲1的學生也會一同查出

以下使用代碼實際驗證一下

創建兩個實體類

學生類

package com.robot.pojo;

public class Student {
    private int id;
    private String name;
    private int tid;

    public Student() {
    }

    public Student(int id, String name, int tid) {
        this.id = id;
        this.name = name;
        this.tid = tid;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getTid() {
        return tid;
    }

    public void setTid(int tid) {
        this.tid = tid;
    }

    @Override
    public String toString() {
        return "Student{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", tid=" + tid +
                '}';
    }
}

老師類，因爲一個老師對應多個學生，所以老師中有學生列表的屬性

package com.robot.pojo;

import java.util.List;

public class Teacher {
    private int id;
    private String name;
    private List<Student> students;

    public Teacher() {
    }

    public Teacher(int id, String name, List<Student> students) {
        this.id = id;
        this.name = name;
        this.students = students;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public List<Student> getStudents() {
        return students;
    }

    public void setStudents(List<Student> students) {
        this.students = students;
    }

    @Override
    public String toString() {
        return "Teacher{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", students=" + students +
                '}';
    }
}

創建接口

創建一個老師的接口，實現根據ID查詢指定老師

package com.robot.dao;

import com.robot.pojo.Teacher;
import org.apache.ibatis.annotations.Param;

public interface TeacherMapper {

    Teacher getTeacher(@Param("tid") int id);
}

因爲是直接查詢老師，然後根據老師的信息來查出學生，沒有直接去查詢學生，所以不用編寫查詢學生的方法

編寫接口對應的xml文件

因爲根據老師ID查詢出的學生是多個，所以用集合標籤< collection >

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

    <select id="getTeacher" resultMap="TeacherToStudent">
        select s.id sid, s.name sname, t.id tid, t.name tname from teacher t, student s where t.id=#{tid} and t.id = s.tid
    </select>
    
    <resultMap id="TeacherToStudent" type="teacher">
        <result property="id" column="tid"/>
        <result property="name" column="tname"/>
        <collection property="students" ofType="student">
            <result property="id" column="sid"/>
            <result property="name" column="sname"/>
            <result property="tid" column="tid"/>
        </collection>
    </resultMap>

</mapper>

編寫核心配置文件

<mappers>
    <mapper resource="com/robot/dao/TeacherMapper.xml"/>
</mappers>

測試

@Test
public void getTeacherTest() {
    SqlSession sqlSession = MybatisUtils.getSqlSession();
    TeacherMapper teacherMapper = sqlSession.getMapper(TeacherMapper.class);
    Teacher teacher = teacherMapper.getTeacher(1);
    System.out.println(teacher);
    sqlSession.close();
}

結果如下

由以上結果可以看到，查出了id=1的老師信息，而和老師關聯的學生也都被查了出來（就是tid=1的學生）