Mysql的左連接與右連接區別

Mysql 的左連接與右連接區別

1.SQL LEFT JOIN 關鍵字

LEFT JOIN 關鍵字會從左表 (table_name1) 那裡返回所有的行，即使在右表 (table_name2) 中沒有匹配的行。

2.SQL RIGHT JOIN 關鍵字

RIGHT JOIN 關鍵字會右表 (table_name2) 那裡返回所有的行，即使在左表 (table_name1) 中沒有匹配的行。

3.在Mysql中的DevDB中創建兩個表

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表中插入數據

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夢','Jone','Jone',NULL);
insert into `Job` (`id`, `FirstName`, `LastName`, `LoginName`, `dep_id`) values('5','Westlife','西城男孩','Westlife','8');

department表數據

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表數據

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夢 Jone Jone (NULL) 5 Westlife 西城男孩 Westlife 8

5.以Job表進行左連接(left join)

/*以Job表進行左連接，此時數據主要以Job表為主，關聯department表，如果Job的dep_id為null或者在department表中
無對應的Id匹配對應的字段自動填充null*/
SELECT * FROM Job table_name1  LEFT JOIN department table_name2 ON table_name1.`dep_id`= table_name2.`ID`;

查出來的數據 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夢 Jone Jone (NULL) (NULL) (NULL) (NULL) 5 Westlife 西城男孩 Westlife 8 (NULL) (NULL) (NULL)

6.以Job表進行右連接(right join)

/*以Job表進行右連接，此時數據主要以department表為主，關聯Job表，如果Job的dep_id為null或者在department表中
無對應的Id匹配對應的字段自動填充null*/
SELECT * FROM Job table_name1  RIGHT JOIN department table_name2 ON table_name1.`dep_id`= table_name2.`ID`;

查出來的數據

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