这篇需结合 <<Mybatis XML 配置文件>>那一篇博客一起看

工作中尽量避免使用多表查询,尤其是对性能要求非常高的项目

我们之前建了个用户表(代码在Mybatis XML配置文件那篇博客里),这次再建一个文章表,代码如下 :

-- 创建⽂章表
DROP TABLE IF EXISTS articleinfo;
CREATE TABLE articleinfo (
 id INT PRIMARY KEY auto_increment,
 title VARCHAR ( 100 ) NOT NULL,
 content TEXT NOT NULL,
 uid INT NOT NULL,
 delete_flag TINYINT ( 4 ) DEFAULT 0 COMMENT '0-正常, 1-删除',
 create_time DATETIME DEFAULT now(),
 update_time DATETIME DEFAULT now() 
) DEFAULT charset 'utf8mb4';
-- 插⼊测试数据
INSERT INTO articleinfo ( title, content, uid ) VALUES( 'Java', 'Java正⽂', 1
);

如何根据文章ID,获取作者的名字和年龄呢?( ta 是表 articleInfo , tb 是表 userinfo )

 select ta.*,tb.username,tb.age from articleinfo ta left join userinfo tb on ta.uid = tb.id where ta.id = 1;

 这样就能拿到文章 id 为1的作者的相关信息了

接下来我们就要跟java联系起来了

先创建一个 java 类,写与 articleinfo 表的字段一一映射的java对象

package com.example.mybatisdemo.model;

import lombok.Data;

import java.util.Date;

@Data
public class ArticleInfo {
    //文章相关信息
    private Integer id;
    private String title;
    private String content;
    private Integer uid;
    private Integer deleteFlag;
    private Date createTime;
    private Date updateTime;
}

然后再创建一个接口,名为ArticleInfoMapper

package com.example.mybatisdemo.mapper;

import com.example.mybatisdemo.model.ArticleInfo;
import org.apache.ibatis.annotations.Mapper;
import org.apache.ibatis.annotations.Select;

@Mapper
public interface ArticleInfoMapper {

    @Select(" select ta.*,tb.username,tb.age from articleinfo ta left join userinfo tb on ta.uid = tb.id where ta.id = #{id}")
    ArticleInfo selectArticleAndUserById(Integer articleId);

}

然后右键,Generate,test,勾选 selectArticleAndUserById ,ok,补充代码

package com.example.mybatisdemo.mapper;

import com.example.mybatisdemo.model.ArticleInfo;
import lombok.extern.slf4j.Slf4j;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

import static org.junit.jupiter.api.Assertions.*;

@Slf4j
@SpringBootTest
class ArticleInfoMapperTest {
    @Autowired
    private ArticleInfoMapper articleInfoMapper;
    @Test
    void selectArticleAndUserById() {
        ArticleInfo articleInfo = articleInfoMapper.selectArticleAndUserById(1);
        log.info(articleInfo.toString());
    }
}

成功 

但是因为 ArticleInfo 并没有 username 和 age ,所以返回对象为 ArticleInfo 的时候是无法返回 username 和 age 的

所以我们就需要在类 ArticleInfo 里面添加 username 和 age

package com.example.mybatisdemo.model;
import lombok.Data;
import java.util.Date;
@Data
public class ArticleInfo {
    //文章相关信息
    private Integer id;
    private String title;
    private String content;
    private Integer uid;
    private Integer deleteFlag;
    private Date createTime;
    private Date updateTime;
    //用户相关信息
    private String username;
    private Integer age;
}

再次运行就能拿到 username 和 age 了