Mysql 的左连接与右连接区别

最新推荐文章于 2025-02-21 10:25:30 发布

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最新推荐文章于 2025-02-21 10:25:30 发布

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原文链接：https://my.oschina.net/qiuzhping/blog/611696

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本文详细解析了Mysql中的左连接和右连接概念，通过实例对比两种连接方式在数据检索上的不同表现，帮助读者理解如何在实际场景中选择合适的连接类型。

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Mysql 的左连接与右连接区别

1.SQL LEFT JOIN 关键字

LEFT JOIN 关键字会从左表 (table_name1) 那里返回所有的行，即使在右表 (table_name2) 中没有匹配的行。

2.SQL RIGHT JOIN 关键字

RIGHT JOIN 关键字会右表 (table_name2) 那里返回所有的行，即使在左表 (table_name1) 中没有匹配的行。

3.在Mysql中的DevDB中创建两个表

Create Table department

CREATE TABLE `department` (
  `ID` int(11) NOT NULL AUTO_INCREMENT,
  `CODE` varchar(100) DEFAULT NULL,
  `NAME` varchar(200) DEFAULT NULL,
  PRIMARY KEY (`ID`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8

Create Table job

CREATE TABLE `job` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `FirstName` varchar(32) DEFAULT NULL,
  `LastName` varchar(32) DEFAULT NULL,
  `LoginName` varchar(32) DEFAULT NULL,
  `dep_id` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8

4.在department表和Job表中插入数据

insert into `department` (`ID`, `CODE`, `NAME`) values('1','Java R&D','Java R&D');
insert into `department` (`ID`, `CODE`, `NAME`) values('2','Mysql R&D','Mysql R&D');
insert into `department` (`ID`, `CODE`, `NAME`) values('3','Interface R&D','Interface R&D');
insert into `department` (`ID`, `CODE`, `NAME`) values('4','Android R&D','Android R&D');
insert into `department` (`ID`, `CODE`, `NAME`) values('5','IOS R&D','IOS R&D');

insert into `Job` (`id`, `FirstName`, `LastName`, `LoginName`, `dep_id`) values('1','Peter','Qiu','Qiu','1');
insert into `Job` (`id`, `FirstName`, `LastName`, `LoginName`, `dep_id`) values('2','Tom','Lee','Lee','2');
insert into `Job` (`id`, `FirstName`, `LastName`, `LoginName`, `dep_id`) values('3','Garfield','Wang','Wang','3');
insert into `Job` (`id`, `FirstName`, `LastName`, `LoginName`, `dep_id`) values('4','哆啦A梦','Jone','Jone',NULL);
insert into `Job` (`id`, `FirstName`, `LastName`, `LoginName`, `dep_id`) values('5','Westlife','西城男孩','Westlife','8');

department表数据

ID	CODE	NAME
1	Java R&D	Java R&D
2	Mysql R&D	Mysql R&D
3	Interface R&D	Interface R&D
4	Android R&D	Android R&D
5	IOS R&D	IOS R&D

Job表数据

id	FirstName	LastName	LoginName	dep_id
1	Peter	Qiu	Qiu	1
2	Tom	Lee	Lee	2
3	Garfield	Wang	Wang	3
4	哆啦A梦	Jone	Jone	(NULL)
5	Westlife	西城男孩	Westlife	8

5.以Job表进行左连接(left join)

/*以Job表进行左连接，此时数据主要以Job表为主，关联department表，如果Job的dep_id为null或者在department表中
无对应的Id匹配对应的字段自动填充null*/
SELECT * FROM Job table_name1  LEFT JOIN department table_name2 ON table_name1.`dep_id`= table_name2.`ID`;

查出来的数据

id	FirstName	LastName	LoginName	dep_id	ID	CODE	NAME
1	Peter	Qiu	Qiu	1	1	Java R&D	Java R&D
2	Tom	Lee	Lee	2	2	Mysql R&D	Mysql R&D
3	Garfield	Wang	Wang	3	3	Interface R&D	Interface R&D
4	哆啦A梦	Jone	Jone	(NULL)	(NULL)	(NULL)	(NULL)
5	Westlife	西城男孩	Westlife	8	(NULL)	(NULL)	(NULL)

6.以Job表进行右连接(right join)

/*以Job表进行右连接，此时数据主要以department表为主，关联Job表，如果Job的dep_id为null或者在department表中
无对应的Id匹配对应的字段自动填充null*/
SELECT * FROM Job table_name1  RIGHT JOIN department table_name2 ON table_name1.`dep_id`= table_name2.`ID`;

查出来的数据

id	FirstName	LastName	LoginName	dep_id	ID	CODE	NAME
1	Peter	Qiu	Qiu	1	1	Java R&D	Java R&D
2	Tom	Lee	Lee	2	2	Mysql R&D	Mysql R&D
3	Garfield	Wang	Wang	3	3	Interface R&D	Interface R&D
(NULL)	(NULL)	(NULL)	(NULL)	(NULL)	4	Android R&D	Android R&D
(NULL)	(NULL)	(NULL)	(NULL)	(NULL)	5	IOS R&D	IOS R&D