Mysql的左连接与右连接区别

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Mysql的左连接与右连接区别

来源: 作者: 时间:2016-01-22 08:34 【

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

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',' 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',' 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 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

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