mybatis 多对一，一对多

准备两张表（class表---student表）

建表脚本

Create Table

CREATE TABLE `tb_class` (
  `id` int NOT NULL AUTO_INCREMENT,
  `code` varchar(18) DEFAULT NULL,
  `NAME` varchar(18) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8



Create Table

CREATE TABLE `tb_student` (
  `id` int NOT NULL AUTO_INCREMENT,
  `NAME` varchar(18) DEFAULT NULL,
  `sex` varchar(18) DEFAULT NULL,
  `age` int DEFAULT NULL,
  `class_id` int DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `class_id` (`class_id`),
  CONSTRAINT `tb_student_ibfk_1` FOREIGN KEY (`class_id`) REFERENCES `tb_class` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=8 DEFAULT CHARSET=utf8

各自表格对应的实体类

package cn.liziy.entity;

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

/**
 * @ClassName Clazz
 * @Author:Liziy
 * @Date 2020/8/6 17:27
 * @Description: 班级实体类
 **/
public class Clazz implements Serializable {
    private static final long serialVersionUID = -8931598000359337543L;
    private Integer id;
    private String code;
    private String name;
    //班级和学生是一对多的关系,一个班级可以有多个学生
    private List<Student> students;

    public Integer getId() {
        return id;
    }

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

    public String getCode() {
        return code;
    }

    public void setCode(String code) {
        this.code = code;
    }

    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;
    }
}

package cn.liziy.entity;

import java.io.Serializable;

/**
 * @ClassName Student
 * @Author:Liziy
 * @Date 2020/8/6 17:29
 * @Description: 学生实体类
 **/
public class Student implements Serializable {
    private static final long serialVersionUID = -1952744231788123787L;
    private Integer id;
    private String name;
    private String sex;
    private Integer age;
    private Clazz clazz;

    public Integer getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

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

    public String getSex() {
        return sex;
    }

    public void setSex(String sex) {
        this.sex = sex;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    public Clazz getClazz() {
        return clazz;
    }

    public void setClazz(Clazz clazz) {
        this.clazz = clazz;
    }
}

多对一（多个学生对应一个班级）

Studentmapper.xml 和 StudentDao 的编写部分

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

<mapper namespace="cn.liziy.dao.StudentDao">
<!-- 查询所有学生信息, 多表连接使用resultMap -->
    <select id="selectAllStu" resultMap="studentResultMap">
        SELECT
                s.id,
                s.name,
                s.sex,
                s.age,
                class_id,
                c.id cid,
                c.code,
                c.name cname
        FROM tb_student s
        LEFT JOIN tb_class c ON class_id = c.id
    </select>

<!-- 映射Student对象的resultMap   -->
    <resultMap id="studentResultMap" type="cn.liziy.entity.Student">
        <id property="id" column="id"/>
        <result property="name" column="name"/>
        <result property="sex" column="sex"/>
        <result property="age" column="age"/>
    <!-- 多对一关联：association       -->
        <association property="clazz" javaType="cn.liziy.entity.Clazz" >
            <id property="id" column="cid"/>
            <result property="code" column="code"/>
            <result property="name" column="cname"/>
        </association>
    </resultMap>

</mapper>

package cn.liziy.dao;

import cn.liziy.entity.Student;

import java.util.List;

public interface StudentDao {


    /**
     * 查询所有学生信息
     * @return
     */
    List<Student> selectAllStu();

}

studentservice层

package cn.liziy.service;

import cn.liziy.entity.Student;

import java.util.List;

/**
 * @ClassName StudentService
 * @Author:Liziy
 * @Date 2020/8/7 12:02
 * @Description:
 **/
public interface StudentService {

    /**
     * 查询所有学生信息
     * @return
     */
    List<Student> selectAllStu();

}

package cn.liziy.service.impl;

import cn.liziy.dao.StudentDao;
import cn.liziy.entity.Student;
import cn.liziy.service.StudentService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

import java.util.List;

/**
 * @ClassName StudentServiceImpl
 * @Author:Liziy
 * @Date 2020/8/7 12:03
 * @Description:
 **/
@Service
public class StudentServiceImpl implements StudentService {

    @Autowired
    StudentDao studentDao;
    @Override
    public List<Student> selectAllStu() {
        return studentDao.selectAllStu();
    }
}

StudentController

package cn.liziy.controller;

import cn.liziy.entity.Student;
import cn.liziy.service.StudentService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.ResponseBody;

import java.util.List;

/**
 * @ClassName StudentController
 * @Author:Liziy
 * @Date 2020/8/7 12:04
 * @Description:
 **/
@Controller
public class StudentController {
    @Autowired
    StudentService studentService;

    /**
     * 查询所有学生信息 多对一
     * @return
     */
    @GetMapping("/students/all")
    public @ResponseBody
    List<Student> studentAll(){
        return studentService.selectAllStu();
    }
}

控制台日志

json数据

每个学生都对应一个班级（多对一）

一对多(一个班级有多个学生)

ClazzMapper和ClazzDao的编写部分

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

<mapper namespace="cn.liziy.dao.ClazzDao">
<!-- 查询所有班级信息, 返回resultMap -->
    <select id="selectAllClazz"  resultMap="clazzMap">
        SELECT
                c.id cid,
                c.code,
                c.name cname,
                s.id,
                s.name,
                s.sex,
                s.age
        FROM tb_class c
        LEFT JOIN tb_student s on  c.id = s.class_id

    </select>
<!--  映射Clazz对象的resultMap  -->
    <resultMap id="clazzMap" type="cn.liziy.entity.Clazz">
        <id property="id" column="cid"/>
        <result property="code" column="code"/>
        <result property="name" column="cname"/>
    <!--  一对多关联映射：collection fetchType="lazy"表示懒加载     -->
        <collection property="students" javaType="ArrayList"
                    column="id" ofType="cn.liziy.entity.Student">
            <id property="id" column="id"/>
            <result property="name" column="name"/>
            <result property="sex" column="sex"/>
            <result property="age" column="age"/>
        </collection>
    </resultMap>
</mapper>

package cn.liziy.dao;

import cn.liziy.entity.Clazz;

import java.util.List;

/**
 * @Author liziyang
 * @Date 22:23 2020/8/6
 * @Description 班级数据访问接口
 **/
public interface ClazzDao {

    /**
     * 查询所有的班级信息
     * @return
     */
    List<Clazz> selectAllClazz();
}

CalzzService

package cn.liziy.service;

import cn.liziy.entity.Clazz;

import java.util.List;

public interface ClazzService {
    /**
     * 查询所有的班级信息
     * @return
     */
    List<Clazz> selectAllClazz();
}

package cn.liziy.service.impl;

import cn.liziy.dao.ClazzDao;
import cn.liziy.entity.Clazz;
import cn.liziy.service.ClazzService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

import java.util.List;

/**
 * @ClassName ClazzServiceImpl
 * @Author:Liziy
 * @Date 2020/8/6 22:40
 * @Description:
 **/
@Service
public class ClazzServiceImpl implements ClazzService {
    @Autowired
    ClazzDao clazzDao;

    @Override
    public List<Clazz> selectAllClazz() {
        return clazzDao.selectAllClazz();
    }
}

ClazzController

package cn.liziy.controller;

import cn.liziy.entity.Clazz;
import cn.liziy.service.ClazzService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.ResponseBody;

import java.util.List;

/**
 * @ClassName ClazzController
 * @Author:Liziy
 * @Date 2020/8/6 22:37
 * @Description:
 **/
@Controller
public class ClazzController {
    @Autowired
    ClazzService clazzService;


    /**
     * 查询所有班级信息 一对多
     * @return
     */
    @GetMapping("/clazz/all")
    public @ResponseBody
    List<Clazz> Clazz(){
        return clazzService.selectAllClazz();
    }
}

控制台日志

json数据

班级和各自班级所属的学生（一对多）