Spring boot2基于Mybatis实现多表关联查询

模拟业务关系：

一个用户user有对应的一个公司company，每个用户有多个账户account。

spring boot 2的环境搭建见上文：spring boot 2整合mybatis

一、mysql创表和模拟数据sql

CREATE TABLE IF NOT EXISTS `user` (
 `id` int(11) NOT NULL AUTO_INCREMENT,
 `name` varchar(50) NOT NULL,
 `company_id` int(11) NOT NULL,
 PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

CREATE TABLE IF NOT EXISTS `company` (
 `id` int(11) NOT NULL AUTO_INCREMENT,
 `name` varchar(200) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

CREATE TABLE IF NOT EXISTS `account` (
 `id` int(11) NOT NULL AUTO_INCREMENT,
 `name` varchar(200) NOT NULL,
 `user_id` int(11) NOT NULL,
 PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

INSERT INTO
  `user`
VALUES
  (1, 'aa', 1),
  (2, 'bb', 2);

INSERT INTO
  `company`
VALUES
  (1, 'xx公司'),
  (2, 'yy公司');

INSERT INTO
  `account`
VALUES
  (1, '中行', 1),
  (2, '工行', 1),
  (3, '中行', 2);

二、创建实体

public class User {
  private Integer id;
  private String name;
  private Company company;
  private List<Account> accounts;
  //getter/setter 这里省略...
}

public class Company {
  private Integer id;
  private String companyName;
    //getter/setter 这里省略...
}

public class Account {
  private Integer id;
  private String accountName;
  //getter/setter 这里省略...

}

三、开发Mapper

方法一：使用注解

1、AccountMapper.java

package com.example.demo.mapper;

import java.util.List;

import org.apache.ibatis.annotations.Result;
import org.apache.ibatis.annotations.Results;
import org.apache.ibatis.annotations.Select;
import com.example.demo.entity.Account;
public interface AccountMapper {
  /*
   * 根据用户id查询账户信息
   */
  @Select("SELECT * FROM `account` WHERE user_id = #{userId}")
  @Results({
    @Result(property = "accountName", column = "name")
  })
  List<Account> getAccountByUserId(Long userId);
}

2、CompanyMapper.java

package com.example.demo.mapper;

import org.apache.ibatis.annotations.Result;
import org.apache.ibatis.annotations.Results;
import org.apache.ibatis.annotations.Select;

import com.example.demo.entity.Company;

public interface CompanyMapper {
  /*
   * 根据公司id查询公司信息
   */
  @Select("SELECT * FROM company WHERE id = #{id}")
  @Results({
    @Result(property = "companyName", column = "name")
  })
  Company getCompanyById(Long id);
}

3、UserMapper.java

package com.example.demo.mapper;

import org.apache.ibatis.annotations.Result;
import org.apache.ibatis.annotations.Results;
import org.apache.ibatis.annotations.Select;
import org.apache.ibatis.annotations.One;
import org.apache.ibatis.annotations.Many;

import com.example.demo.entity.User;

public interface UserMapper {

  /*
   * 一对一查询
   * property：查询结果赋值给此实体属性
   * column：对应数据库的表字段，做为下面@One(select方法的查询参数
   * one：一对一的查询
   * @One(select = 方法全路径) ：调用的方法
   */
  @Select("SELECT * FROM user WHERE id = #{id}")
  @Results({
    @Result(property = "company", column = "company_id", one = @One(select = "com.example.demo.mapper.CompanyMapper.getCompanyById"))
  })
  User getUserWithCompany(Long id);

  /*
   * 一对多查询
   * property：查询结果赋值给此实体属性
   * column：对应数据库的表字段，可做为下面@One(select方法)的查询参数
   * many：一对多的查询
   * @Many(select = 方法全路径) ：调用的方法
   */
  @Select("SELECT * FROM user WHERE id = #{id}")
  @Results({
    @Result(property = "id", column = "id"),//加此行，否则id值为空
    @Result(property = "accounts", column = "id", many = @Many(select = "com.example.demo.mapper.AccountMapper.getAccountByUserId"))
  })
  User getUserWithAccount(Long id);

  /*
   * 同时用一对一、一对多查询
   */
  @Select("SELECT * FROM user")
  @Results({
    @Result(property = "id", column = "id"),
    @Result(property = "company", column = "company_id", one = @One(select = "com.example.demo.mapper.CompanyMapper.getCompanyById")),
    @Result(property = "accounts", column = "id", many = @Many(select = "com.example.demo.mapper.AccountMapper.getAccountByUserId"))
  })
  List<User> getAll();
}

方法二：使用XML

参考上文spring boot 2整合mybatis配置application.properties和mybatis-config.xml等后，
以上面的getAll()方法为例，UserMapper.xml配置如下：

<?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.example.demo.mapper.UserMapper" >
  <resultMap id="UserMap" type="com.example.demo.entity.User">
    <id column="id" jdbcType="INTEGER" property="id" />
    <result property="name" column="name" jdbcType="VARCHAR" />
    <!--封装映射company表数据，user表与company表1对1关系，配置1对1的映射
      association:用于配置1对1的映射
            属性property：company对象在user对象中的属性名
            属性javaType：company属性的java对象 类型
            属性column：user表中的外键引用company表
    -->
    <association property="company" javaType="com.example.demo.entity.Company" column="company_id">
      <id property="id" column="companyid"></id>
      <result property="companyName" column="companyname"></result>
    </association>
    <!--配置1对多关系映射
      property：在user里面的List<Account>的属性名
      ofType:当前account表的java类型
      column:外键
    -->
    <collection property="accounts" ofType="com.example.demo.entity.Account" column="user_id">
      <id property="id" column="accountid"></id>
      <result property="accountName" column="accountname"></result>
    </collection>
   </resultMap>

  <select id="getAll" resultMap="UserMap" >
    SELECT
    u.id,u.name,c.id companyid, c.name companyname, a.id accountid,a.name accountname
    FROM user u
    LEFT JOIN company c on u.company_id=c.id
    LEFT JOIN account a on u.id=a.user_id
  </select>
</mapper>

四、控制层

package com.example.demo.web;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

import com.example.demo.entity.User;
import com.example.demo.mapper.UserMapper;

@RestController
public class UserController {
  @Autowired
  private UserMapper userMapper;

  //请求例子：http://localhost:9001/getUserWithCompany/1
  /*请求结果：{"id":1,"name":"aa","company":{"id":1,"companyName":"xx公司"},"accounts":null}*/
  @RequestMapping("/getUserWithCompany/{id}")
  public User getUserWithCompany(@PathVariable("id") Long id) {
    User user = userMapper.getUserWithCompany(id);
    return user;
  }

  //请求例子：http://localhost:9001/getUserWithAccount/1
  /*请求结果：{"id":1,"name":"aa","company":null,"accounts":[{"id":1,"accountName":"中行"},{"id":2,"accountName":"工行"}]}*/
  @RequestMapping("/getUserWithAccount/{id}")
  public User getUserWithAccount(@PathVariable("id") Long id) {
    User user = userMapper.getUserWithAccount(id);
    return user;
  }

  //请求例子：http://localhost:9001/getUserWithAccount/1
  /*请求结果：[{"id":1,"name":"aa","company":{"id":1,"companyName":"xx公司"},"accounts":[{"id":1,"accountName":"中行"},
    {"id":2,"accountName":"工行"}]},{"id":2,"name":"bb","company":{"id":2,"companyName":"yy公司"},"accounts":[{"id":3,"accountName":"中行"}]}]*/
  @RequestMapping("/getUsers")
  public List<User> getUsers() {
    List<User> users=userMapper.getAll();
    return users;
  }
}

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