Mybatis入门(六)联查之一对多

上一章说了多对一，很多学生被一个老师教，这一章是一个老师教很多学生

目录基本没有变化只是改了配置文件：

2、配置文件：

TeacherMapper接口类：

package com.hdlf.dao;

import com.hdlf.pojo.student;
import com.hdlf.pojo.teacher;

import java.util.List;

public interface TeacherMapper {
    //方式一
    teacher getteacher(int tid);
    //方式二
    teacher getteacher2(int tid);

}

teacher实体类：

package com.hdlf.pojo;

import lombok.Data;

import java.util.List;

public class teacher {
    private int tid;
    private String tname;
    //因为需要获取到学生所以在这里加一个List泛型集合
    List<student> students;

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

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

    public teacher() {
    }

    public teacher(int tid, String tname, List<student> students) {
        this.tid = tid;
        this.tname = tname;
        this.students = students;
    }

    public int getTid() {
        return tid;
    }

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

    public String getTname() {
        return tname;
    }

    public void setTname(String tname) {
        this.tname = tname;
    }

    @Override
    public String toString() {
        return "teacher{" +
                "tid=" + tid +
                ", tname=‘" + tname + ‘\‘‘ +
                ", students=" + students +
                ‘}‘;
    }
}

student实体类因为是根据老师查教过多少学生，学生类不用写特别的类：

package com.hdlf.pojo;

public class student {
    private int sid;
    private String sname;
    private int tid;

    @Override
    public String toString() {
        return "student{" +
                "sid=" + sid +
                ", sname=‘" + sname + ‘\‘‘ +
                ", tid=" + tid +
                ‘}‘;
    }

    public int getSid() {
        return sid;
    }

    public void setSid(int sid) {
        this.sid = sid;
    }

    public String getSname() {
        return sname;
    }

    public void setSname(String sname) {
        this.sname = sname;
    }

    public int getTid() {
        return tid;
    }

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

    public student(int sid, String sname, int tid) {
        this.sid = sid;
        this.sname = sname;
        this.tid = tid;
    }

    public student() {
    }
}

TeacherMapper.xml配置类:

这个是方式一：

复杂的属性，需要单独处理 对象使用：association

集合使用：collection

javaType 指定属性类型，集合中泛型信息使用ofType获取

<?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.hdlf.dao.TeacherMapper">

    <select id="getteacher" resultMap="getstudent">
        select t.tid, t.tname ,s.sname,s.sid from student s,teacher t where s.tid = t.tid and t.tid = #{tid};
    </select>

    <resultMap id="getstudent" type="teacher">
        <result property="tid" column="tid"></result>
        <result property="tname" column="tname"></result>

        <!--复杂的属性，需要单独处理 对象：association 集合：collection
        javaType 指定属性类型
        集合中泛型信息使用ofType获取
        这个studens就是teacher实体类中的students-->
        <collection property="students" ofType="student">
            <result property="sid" column="sid"></result>
            <result property="sname" column="sname"></result>
        </collection>

    </resultMap>
</mapper>

测试结果：

teacher{tid=0, tname=‘王老师‘, students=[student{sid=1, sname=‘张三‘, tid=1}, student{sid=2, sname=‘李四‘, tid=1}, student{sid=3, sname=‘王五‘, tid=1}, student{sid=4, sname=‘吴六‘, tid=1}, student{sid=5, sname=‘赵七‘, tid=1}]}

方式二个人理解其实都一样：

<?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.hdlf.dao.TeacherMapper">
  <select id="getteacher2" resultMap="getallteacher">
        select * from mybatis.teacher where tid = #{tid}
    </select>
    <resultMap id="getallteacher" type="teacher">
        <collection property="students" select="getallstudent" javaType="ArrayList" ofType="studnet" column="tid"></collection>
    </resultMap>
    <select id="getallstudent" resultType="student">
        select * from  mybatis.student;
    </select>

</mapper>

这个就是一对多的处理，小编还是没有领悟到其中的精髓，希望过几天可以顿悟。

原文地址：https://www.cnblogs.com/jzfanqiejiang/p/12207503.html