【Spring Boot】MyBatis多表查询的操作：注解和XML实现SQL语句

1.准备工作

1.1创建数据库

（1）创建数据库：

CREATE DATABASE mybatis_test DEFAULT CHARACTER SET utf8mb4;

（2）使用数据库

-- 使⽤数据数据
USE mybatis_test;

1.2 创建用户表和实体类

创建用户表

-- 创建表[⽤⼾表]CREATE TABLE `user_info` (`id` INT ( 11 ) NOT NULL AUTO_INCREMENT,`username` VARCHAR ( 127 ) NOT NULL,`password` VARCHAR ( 127 ) NOT NULL,`age` TINYINT ( 4 ) NOT NULL,`gender` TINYINT ( 4 ) DEFAULT '0' COMMENT '1-男 2-⼥ 0-默认',`phone` VARCHAR ( 15 ) DEFAULT NULL,`delete_flag` TINYINT ( 4 ) DEFAULT 0 COMMENT '0-正常, 1-删除',`create_time` DATETIME DEFAULT now(),`update_time` DATETIME DEFAULT now() ON UPDATE now(),PRIMARY KEY ( `id` ) 
) ENGINE = INNODB DEFAULT CHARSET = utf8mb4; 

添加用户信息

-- 添加⽤⼾信息
INSERT INTO user_info( username, `password`, age, gender, phone )
VALUES ( 'admin', 'admin', 18, 1, '18612340001' );
INSERT INTO user_info( username, `password`, age, gender, phone )
VALUES ( 'zhangsan', 'zhangsan', 18, 1, '18612340002' );
INSERT INTO user_info( username, `password`, age, gender, phone )
VALUES ( 'lisi', 'lisi', 18, 1, '18612340003' );
INSERT INTO user_info( username, `password`, age, gender, phone )
VALUES ( 'wangwu', 'wangwu', 18, 1, '18612340004' );

实体类的属性名与表中的字段名⼀⼀对应

@Data
public class UserInfo {private Integer id;private String username;private String password;private Integer age;private Integer gender;private String phone;private Integer deleteFlag;private Date createTime;private Date updateTime;}

1.3 创建文章表和实体类

上⾯建了⼀张⽤⼾表, 我们再来建⼀张⽂章表, 进⾏多表关联查询.
⽂章表的uid, 对应⽤⼾表的id.

创建文章表：

-- 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';

插入数据

-- 插⼊测试数据
INSERT INTO articleinfo ( title, content, uid ) VALUES ( 'Java', 'Java正文', 1
);
INSERT INTO articleinfo ( title, content, uid ) VALUES ( 'MySQL', 'MySQL正文', 1
);
INSERT INTO articleinfo ( title, content, uid ) VALUES ( 'C', 'C正文', 2
);

实体类的属性名与表中的字段名⼀⼀对应

@Data
public class ArticleInfo { private Integer id; private String title; private String content; private Integer uid; private Integer deleteFlag; private Date createTime; private Date updateTime;
}

1.4 配置文件

在Application.yml文件中配置：

# 数据库连接配置
spring:datasource:# MySQL在远程服务器上url: jdbc:mysql://x.x.x.x:3306/mybatis_test?characterEncoding=utf8&useSSL=falseusername: root  #MySQL账号password: root  #MySQL密码driver-class-name: com.mysql.cj.jdbc.Drivermybatis:configuration: # 配置打印 MyBatis⽇志log-impl: org.apache.ibatis.logging.stdout.StdOutImplmap-underscore-to-camel-case: true #配置驼峰⾃动转换# 配置 mybatis xml 的⽂件路径，在 resources/mapper 创建所有表的 xml ⽂件mapper-locations: classpath:mapper/**Mapper.xml

2.多表查询

2.1 需求: 根据uid查询书的作者等相关信息

2.1.1 注解实现

SQL命令：

SELECTa.id,a.title,a.content,a.uid,b.username,b.age,b.gender
FROMarticleinfo aLEFT JOIN user_info b ON a.uid = b.id
WHEREa.id =1;

根据查询的结果在ArticleInfo 类补充相关的属性：

@Data
public class ArticleInfo {private Integer id;private String title;private String content;private Integer uid;private Integer deleteFlag;private Date createTime;private Date updateTime;// 补充⽤⼾相关信息private String username;private Integer age;private Integer gender;}

ArticleInfoMapper接口：

@Mapper
public interface ArticleInfoMapper {@Select("select a.id,a.title,a.content,a.uid, b.username, b.age, b.gender " + // 注意最后的空格"from articleinfo a left join user_info b on a.uid=b.id " +"where a.id = #{id}")ArticleInfo queryArticleAndUser(Integer id);
}

测试代码：

@Slf4j
@SpringBootTest
class ArticleInfoMapperTest {@Autowiredprivate ArticleInfoMapper articleInfoMapper;@Testvoid queryArticleAndUser() {articleInfoMapper.queryArticleAndUser(1);}
}

运行结果：

如果名称不⼀致的, 采⽤Results, 或者别名的⽅式解决, 和单表查询⼀样 Mybatis 不分单表还是多表, 主要就是三部分: SQL, 映射关系和实体类

2.1.2 XML实现

SQL命令：

SELECTa.id,a.title,a.content,a.uid,b.username,b.age,b.gender
FROMarticleinfo aLEFT JOIN user_info b ON a.uid = b.id
WHEREa.id =1;

根据查询的结果在ArticleInfo 类补充相关的属性：

@Data
public class ArticleInfo {private Integer id;private String title;private String content;private Integer uid;private Integer deleteFlag;private Date createTime;private Date updateTime;// 补充⽤⼾相关信息private String username;private Integer age;private Integer gender;}

ArticleInfoMapper接口：

@Mapper
public interface ArticleInfoXMLMapper {ArticleInfo queryArticleAndUser(Integer id);
}

ArticleInfoXMLMapper.xml文件：

<?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="org.example.mybatis.mapper.ArticleInfoXMLMapper"><select id ="queryArticleAndUser" resultType="org.example.mybatis.model.ArticleInfo">select a.id,a.title,a.content,a.uid, b.username, b.age, b.genderfrom articleinfo a left join user_info b on a.uid=b.idwhere a.id = #{id}</select></mapper>

测试代码：

@Slf4j
@SpringBootTest
class ArticleInfoXMLMapperTest {@Autowiredprivate ArticleInfoXMLMapper articleInfoXMLMapper;@Testvoid queryArticleAndUser() {articleInfoXMLMapper.queryArticleAndUser(1);}
}

运行结果：

如果名称不⼀致的, 采⽤ResultMap, 或者别名的⽅式解决, 和单表查询⼀样 Mybatis 不分单表还是多表, 主要就是三部分: SQL, 映射关系和实体类

2.2 需求: 根据user_in的id查询作者创作的书相关信息

2.1.1 注解实现

SQL命令：

SELECTb.username,b.age,b.gender,a.id,a.title,a.content,a.uid
FROMarticleinfo aRIGTH JOIN user_info  b ON b.uid = a.id
WHEREb.id =1;

根据查询的结果在ArticleInfo 类补充相关的属性：

@Data
public class ArticleInfo {private Integer id;private String title;private String content;private Integer uid;private Integer deleteFlag;private Date createTime;private Date updateTime;// 补充⽤⼾相关信息private String username;private Integer age;private Integer gender;}

ArticleInfoMapper接口：

@Mapper
public interface ArticleInfoMapper {@Select("select b.username, b.age,b.gender,a.id,a.title,a.content,a.uid " + // 注意最后的空格"from articleinfo a right join user_info b on a.uid=b.id " +"where b.id = #{id}")List<ArticleInfo> queryArticleAndUser(Integer id);
}

测试代码：

@Slf4j
@SpringBootTest
class ArticleInfoMapperTest {@Autowiredprivate ArticleInfoMapper articleInfoMapper;@Testvoid queryArticleAndUser() {articleInfoMapper.queryArticleAndUser(1);}
}

运行结果：

如果名称不⼀致的, 采⽤Results, 或者别名的⽅式解决, 和单表查询⼀样 Mybatis 不分单表还是多表, 主要就是三部分: SQL, 映射关系和实体类

2.1.2 XML实现

SQL命令：

SELECTb.username,b.age,b.gender,a.id,a.title,a.content,a.uid
FROMarticleinfo aRIGHT JOIN user_info b ON a.uid = b.id
WHEREa.id =1;

根据查询的结果在ArticleInfo 类补充相关的属性：

@Data
public class ArticleInfo {private Integer id;private String title;private String content;private Integer uid;private Integer deleteFlag;private Date createTime;private Date updateTime;// 补充⽤⼾相关信息private String username;private Integer age;private Integer gender;}

ArticleInfoMapper接口：

@Mapper
public interface ArticleInfoXMLMapper {List<ArticleInfo> queryArticleAndUser(Integer id);
}

ArticleInfoXMLMapper.xml文件：

<?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="org.example.mybatis.mapper.ArticleInfoXMLMapper"><select id ="queryArticleAndUser" resultType="org.example.mybatis.model.ArticleInfo">select b.username, b.age, b.gender,a.id,a.title,a.content,a.uidfrom articleinfo a right join user_info b on a.uid=b.idwhere b.id = #{id}</select></mapper>

测试代码：

@Slf4j
@SpringBootTest
class ArticleInfoXMLMapperTest {@Autowiredprivate ArticleInfoXMLMapper articleInfoXMLMapper;@Testvoid queryArticleAndUser() {articleInfoXMLMapper.queryArticleAndUser(1);}
}

运行结果：

如果名称不⼀致的, 采⽤ResultMap, 或者别名的⽅式解决, 和单表查询⼀样 Mybatis 不分单表还是多表, 主要就是三部分: SQL, 映射关系和实体类