mysql 右连接(right join)

0.前言

前面我们已经简单介绍过mysql的左连接left join。这篇文章简单介绍下mysql的右连接right join。

1.简单使用

“右连接”，表1右连接表2，以右为主，表示以表2为主，关联查询表1的数据，查出表2所有数据以及表1和表2有交集的数据，如下：

DROP TABLE IF EXISTS `students`;
CREATE TABLE `students` (
    `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
    `stu_name` varchar(20) NOT NULL ,
    `stu_sex` varchar(10)  NOT NULL ,
    `class_id` int(10)  NOT NULL ,
    `age` int(10) NOT NULL  DEFAULT 18,
    `create_time` datetime NULL DEFAULT CURRENT_TIMESTAMP,
    `update_time` datetime NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
    PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1000 DEFAULT CHARSET=utf8;



insert into students (stu_name,stu_sex,class_id,age) values
('张三','男',1001,18),
('王晓红','女',2001,20),
('李清乐','男',1001,23),
('赵倚天','男',3001,23),
('赵四','男',1001,22),
('王燕','女',3001,21),
('李西西','女',2001,20),
('高严','男',3001,22);

DROP TABLE IF EXISTS `teacher`;
CREATE TABLE `teacher` (
    `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
    `teacher_name` varchar(20) NOT NULL ,
    `teacher_sex` varchar(10)  NOT NULL ,
    `class_id` int(10)  NOT NULL ,
    `age` int(10) NOT NULL  DEFAULT 18,
		`tel` varchar(11) ,
    `create_time` datetime NULL DEFAULT CURRENT_TIMESTAMP,
    `update_time` datetime NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
    PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1000 DEFAULT CHARSET=utf8;


insert into teacher (teacher_name,teacher_sex,class_id,age,tel) values
('王老师','男',1001,38,'13011112222'),
('李老师','男',2001,28,'13012344222'),
('赵老师','男',3001,38,'13011112233'),
('孙老师','男',4001,28,'14011112222'),
('周老师','男',5001,18,' ');

表1：

表2：

我们执行右连接：

select * 
from students as s
right join teacher as t
on s.class_id = t.class_id

结果：

可以看出，数据返回的结果是以表2（也就是右表）为主，即时出现空的对于也会正常显示。

2.扩展

如果我们只要最右边部分，则执行下面的语句。

select * 
from students as s
right join teacher as t
on s.class_id = t.class_id
where s.class_id is null

结果：