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

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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)

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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)

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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