spring boot 2使用Mybatis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;
    }    
}