MyBatis多表關聯查詢

阿新 • • 發佈：2021-08-10

一、搭建測試環境（一對多和多對一）

（一）建立測試資料庫

CREATE TABLE `t_teacher` (
 `id` int(10) NOT NULL AUTO_INCREMENT,
 `name` varchar(30) DEFAULT NULL,
 PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8

CREATE TABLE `t_student` (
 `id` int(10) NOT NULL AUTO_INCREMENT,
 `name` varchar(30) DEFAULT NULL,
 `tid` int(10) DEFAULT NULL,
 PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8

（二）根據測試資料庫建立實體類

學生類

package com.jarreet.test.pojo;

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;
import lombok.ToString;

@Data
@NoArgsConstructor
@AllArgsConstructor
@ToString
public class Student {
  private Integer id;
  private String name;
  private Integer tid;

  // 學生關聯一個老師
  private Teacher teacher;
}

老師類

package com.jarreet.test.pojo;

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;
import lombok.ToString;

import java.util.ArrayList;
import java.util.List;

@Data
@AllArgsConstructor
@NoArgsConstructor
@ToString
public class Teacher {
  private Integer id;
  private String name;

  // 老師關聯一群學生
  private List<Student> studentList = new ArrayList<>();
}

（三）檔案配置

在 application.yml 配置 mybatis

mybatis:
  type-aliases-package: com.jarreet.test.pojo
  mapper-locations: classpath:mapper/*.xml
  configuration:
   map-underscore-to-camel-case: true

在 TestApplication 啟動類上加上包掃描註解
```
@MapperScan("com.jarreet.test.dao")
```

二、多對一查詢

我們根據上面搭建的環境，給出一個查詢需求：查詢所有學生的資訊及他們的老師

（一）資料持久化層

StudentDao.java

package com.jarreet.test.dao;

import com.jarreet.test.pojo.Student;
import org.springframework.stereotype.Repository;

import java.util.List;

@Repository
public interface StudentDao {

  public List<Student> getAllStudents();
}

StudentMapper.xml

<?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="com.jarreet.test.dao.StudentDao">

  <!-- 查詢文章管理列表多對一配置 -->
  <resultMap id="studentAndTeacher" type="com.jarreet.test.pojo.Student">
    <id property="id" column="sid"/>
    <result property="name" column="sname"/>
    <result property="tid" column="stid"/>
    <association property="teacher" javaType="com.jarreet.test.pojo.Teacher">
      <id property="id" column="tid"/>
      <result property="name" column="tname"/>
    </association>
  </resultMap>
  <!--
    查詢學生列表
    注意：要給屬性取別名，
    因為老師和學生都有id和name屬性，
    若不取別名則會衝突覆蓋
  -->
  <select id="getAllStudents" resultMap="studentAndTeacher">
     select
        s.id sid,
        s.name sname,
        s.tid stid,
        t.id tid,
        t.name tname
     from
       t_student s, t_teacher t
     where
        s.tid = t.id
  </select>
</mapper>

（二）業務邏輯層

StudentService.java

package com.jarreet.test.service;

import com.jarreet.test.pojo.Student;

import java.util.List;

public interface StudentService {

  List<Student> getAllStudents();
}

StudentServiceImpl.java

package com.jarreet.test.service.impl;

import com.jarreet.test.dao.StudentDao;
import com.jarreet.test.pojo.Student;
import com.jarreet.test.service.StudentService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

import java.util.List;

@Service
public class StudentServiceImpl implements StudentService {

  @Autowired
  private StudentDao studentDao;

  @Override
  public List<Student> getAllStudents() {
    return studentDao.getAllStudents();
   }
}

（三）訪問控制層

StudentController.java

package com.jarreet.test.controller;

import com.jarreet.test.pojo.Student;
import com.jarreet.test.service.StudentService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.ResponseBody;

import java.util.List;

@Controller
public class StudentController {

  @Autowired
  private StudentService studentService;

  @RequestMapping("/student")
  @ResponseBody
  public String studentList() {
    List<Student> allStudents = studentService.getAllStudents();
    return allStudents.toString();
   }
}

（四）結果展示

三、一對多查詢

我們根據上面搭建的環境，給出一個查詢需求：查詢所有老師的資訊及他們的學生

（一）資料持久化層

TeacherDao.java

package com.jarreet.test.dao;

import com.jarreet.test.pojo.Teacher;
import org.springframework.stereotype.Repository;

import java.util.List;

@Repository
public interface TeacherDao {

  public List<Teacher> getAllTeachers();
}

TeacherMapper.xml

<?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="com.jarreet.test.dao.TeacherDao">

  <!-- 查詢老師列表一對多配置 -->
  <resultMap id="teacherAndStudent" type="com.jarreet.test.pojo.Teacher">
    <id property="id" column="tid"/>
    <result property="name" column="tname"/>
    <collection property="studentList" ofType="com.jarreet.test.pojo.Student">
      <id property="id" column="sid"/>
      <result property="name" column="sname"/>
      <result property="tid" column="stid"/>
    </collection>
  </resultMap>
  <!--  查詢老師列表  -->
  <select id="getAllTeachers" resultMap="teacherAndStudent">
     select
        t.id tid,
        t.name tname,
        s.id sid,
        s.name sname,
        s.tid stid
     from
       t_teacher t, t_student s
     where
        t.id = s.tid
  </select>
</mapper>

（二）業務邏輯層

TeacherService.java

package com.jarreet.test.service;

import com.jarreet.test.pojo.Teacher;

import java.util.List;

public interface TeacherService {
  
  List<Teacher> getAllTeachers();
}

TeacherServiceImpl.java

package com.jarreet.test.service.impl;

import com.jarreet.test.dao.TeacherDao;
import com.jarreet.test.pojo.Teacher;
import com.jarreet.test.service.TeacherService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

import java.util.List;

@Service
public class TeacherServiceImpl implements TeacherService {
  
  @Autowired
  private TeacherDao teacherDao;
  
  @Override
  public List<Teacher> getAllTeachers() {
    return teacherDao.getAllTeachers();
   }
}

（三）訪問控制層

TeacherController.java

package com.jarreet.test.controller;

import com.jarreet.test.pojo.Teacher;
import com.jarreet.test.service.TeacherService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.ResponseBody;

import java.util.List;

@Controller
public class TeacherController {

  @Autowired
  private TeacherService teacherService;

  @RequestMapping("/teacher")
  @ResponseBody
  public String teacherList() {
    List<Teacher> allTeachers = teacherService.getAllTeachers();
    return allTeachers.toString();
   }
}

（四）結果展示

四、多對多查詢

多對多查詢其實就是雙向的一對多查詢

這裡我們給出一個多對多的情境：網上選課，一個賬號可以選擇多個課程，而一個課程也有可能被多個賬號選擇

那麼我們現在需要一個賬號表 t_account ，一個課程表 t_course；但是這兩張表就夠了嗎？

當然不夠，開頭說了，多對多查詢是雙向的一對多，那麼我們就需要一張中間表 account_course 來實現

（一）建立資料庫表

CREATE TABLE `t_account`(
`id` INT(11) PRIMARY KEY AUTO_INCREMENT,
`name` VARCHAR(11) DEFAULT NULL
);

CREATE TABLE `t_course`(
`id` INT(11) PRIMARY KEY AUTO_INCREMENT,
`name` VARCHAR(11) DEFAULT NULL
);

CREATE TABLE `account_course`(
`id` INT(11) PRIMARY KEY AUTO_INCREMENT,
`aid` INT(11) DEFAULT NULL,
`cid` INT(11) DEFAULT NULL
);

（二）根據資料庫表建立實體類

賬戶類

package com.jarreet.test.pojo;

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;
import lombok.ToString;

import java.util.List;

@Data
@NoArgsConstructor
@AllArgsConstructor
@ToString
public class Account {

  private Integer id;
  private String name;
  private List<Course> courseList;
}

課程類

package com.jarreet.test.pojo;

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;
import lombok.ToString;

import java.util.List;

@Data
@NoArgsConstructor
@AllArgsConstructor
@ToString
public class Course {
  
  private Integer id;
  private String name;
  private List<Account> accountList;
}

（三）資料持久化層

AccountDao.java

package com.jarreet.test.dao;

import com.jarreet.test.pojo.Account;
import org.springframework.stereotype.Repository;

@Repository
public interface AccountDao {
  public Account findById(Integer id);
}

AccountMapper.xml

<?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="com.jarreet.test.dao.AccountDao">
  
  <resultMap id="accountMap" type="com.jarreet.test.pojo.Account">
    <id property="id" column="aid"/>
    <result property="name" column="aname"/>
    <collection property="courseList" ofType="com.jarreet.test.pojo.Course">
      <id property="id" column="cid"/>
      <result property="name" column="cname"/>
    </collection>
  </resultMap>
  
  <select id="findById" resultMap="accountMap">
     select
        a.id aid,
        a.name aname,
        c.id cid,
        c.name cname
     from
       t_account a, t_course c, account_course ac
     where
        a.id = #{id}
      and a.id = ac.aid
      and c.id = ac.cid
  </select>
</mapper>

CourseDao.java

package com.jarreet.test.dao;

import com.jarreet.test.pojo.Course;
import org.springframework.stereotype.Repository;

@Repository
public interface CouseDao {
  public Course findById(Integer id);
}

CourseMapper.xml

<?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="com.jarreet.test.dao.CouseDao">

  <resultMap id="courseMap" type="com.jarreet.test.pojo.Course">
    <id property="id" column="cid"/>
    <result property="name" column="cname"/>
    <collection property="accountList" ofType="com.jarreet.test.pojo.Account">
      <id property="id" column="aid"/>
      <result property="name" column="aname"/>
    </collection>
  </resultMap>

  <select id="findById" resultMap="courseMap">
     select
       c.id cid,
       c.name cname,
       a.id aid,
       a.name aname
     from
       t_account a, t_course c, account_course ac
     where
       c.id = #{id}
      and a.id = ac.aid
      and c.id = ac.cid
  </select>
</mapper>

（四）業務邏輯層

AccountService.java

package com.jarreet.test.service;

import com.jarreet.test.pojo.Account;

public interface AccountService {

  Account findAccountById(Integer id);
}

AccountServiceImpl.java

package com.jarreet.test.service.impl;

import com.jarreet.test.dao.AccountDao;
import com.jarreet.test.pojo.Account;
import com.jarreet.test.service.AccountService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

@Service
public class AccountServiceImpl implements AccountService {
  
  @Autowired
  private AccountDao accountDao;
  
  @Override
  public Account findAccountById(Integer id) {
    return accountDao.findById(id);
   }
}

CourseService.java

package com.jarreet.test.service;

import com.jarreet.test.pojo.Course;

public interface CourseService {
  
  Course findCourseById(Integer id);
}

CourseServiceImpl.java

package com.jarreet.test.service.impl;

import com.jarreet.test.dao.CourseDao;
import com.jarreet.test.pojo.Course;
import com.jarreet.test.service.CourseService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

@Service
public class CourseServiceImpl implements CourseService {
  
  @Autowired
  private CourseDao courseDao;

  @Override
  public Course findCourseById(Integer id) {
    return courseDao.findById(id);
   }
}

（五）訪問控制層

AccountController.java

package com.jarreet.test.controller;

import com.jarreet.test.pojo.Account;
import com.jarreet.test.service.AccountService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.ResponseBody;

@Controller
public class AccountController {

  @Autowired
  private AccountService accountService;

  @RequestMapping("/account/{id}")
  @ResponseBody
  public String findAccountById(@PathVariable Integer id) {
    Account account = accountService.findAccountById(id);
    return account.toString();
   }
}

CourseController.java

package com.jarreet.test.controller;

import com.jarreet.test.pojo.Course;
import com.jarreet.test.service.CourseService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.ResponseBody;

@Controller
public class CourseController {

  @Autowired
  private CourseService courseService;

  @RequestMapping("/course/{id}")
  @ResponseBody
  public String findCourseById(@PathVariable Integer id) {
    Course course = courseService.findCourseById(id);
    return course.toString();
   }
}

（六）結果展示

至此，關於 Mybatis 的多表查詢就講解完畢了，下面我們簡單做個總結。

Mybatis的多表查詢無非就是做好實體類屬性和mapper.xml檔案的對映，其中：

association 是將結果集封裝成⼀個實體類的物件（⼀個⽬標物件），並通過 JavaType 設定數型別；

collection 是將結果集封裝成⼀個集合物件（多個⽬標物件），並通過 ofType 設定資料型別；

當多對一時，使用 association

當一對多時，使用 collection；

而當多對多時，無非就是進行一個雙向的一對多繫結

特別要注意的幾個點：

搭建環境時，要在配置檔案中配置好 mybatis 資訊，否則有可能報錯

需要開啟包掃描，要麼在啟動類上加上 @MapperScan 註解，要麼在持久化介面上加上 @Mapper 註解

當需要級聯的物件的屬性名和自身的屬性名相同時，需要起別名，防止衝突覆蓋

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