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。

巴特西

Mybatis入门(六)联查之一对多

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