一对多处理

一个老师拥有多个学生（对老师而言，就是一对多的关系）

1、新建实体类

Teacher

package com.sw.pojo;

import lombok.Data;

import java.io.Serializable;
import java.util.List;

/**
 * @Author suaxi
 * @Date 2020/12/13 10:55
 */
@Data
public class Teacher implements Serializable {

    private int id;
    private String name;
    //一个老师对应多个学生
    private List<Student> students;
}

Student

package com.sw.pojo;

import lombok.Data;

import java.io.Serializable;

/**
 * @Author suaxi
 * @Date 2020/12/13 10:55
 */
@Data
public class Student implements Serializable {

    private int id;
    private String name;
    private int tid;
}

2、TeacherMapper.xml配置（两种方式）

<?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.sw.dao.TeacherMapper">

    <!--按结果集嵌套处理-->
    <select id="findTeacherById" resultMap="TeacherStudent">
        select s.id sid,s.name sname,t.id tid ,t.name tname
        from student s,teacher t
        where s.tid = t.id and t.id = #{tid};
    </select>
    
    <resultMap id="TeacherStudent" type="teacher">
        <result property="id" column="tid"/> 
        <result property="name" column="tname"/>
        <!--
        集合使用 collection
        javaType="" 指定属性的类型
        集合中的泛型信息，使用 ofType 获取
        -->
        <collection property="students" ofType="Student">
            <result property="id" column="sid"/>
            <result property="name" column="sname"/>
            <result property="tid" column="tid"/>
        </collection>
    </resultMap>

    <!--按照查询嵌套处理-->
    <select id="findTeacherById2" resultMap="TeacherStudent2">
        select * from teacher where id = #{tid};
    </select>
    <resultMap id="TeacherStudent2" type="Teacher">
        <collection property="students" javaType="ArrayList" ofType="Student" select="findStudentByTeacherId" column="id"/>
    </resultMap>

    <select id="findStudentByTeacherId" resultType="Student">
        select * from student where tid = #{tid};
    </select>
</mapper>

注：

1、关联 - association 【多对一】

2、集合 - collection 【一对多】

3、javaType & ofType

• javaType用来指定实体类中属性的类型
• ofType用来指定映射到List或者集合中的pojo类型，泛型中的约束类型