数据库的左连接，右连接，内链接:

Posted 2020-09-05 Yi_Warmth

先建立数据库，代码如下：

create database if not exists "emp";

use emp;

drop table if exists "dept";

create table dept( `depid` varchar(50) NOT NULL,    `deptname` varchar(100) default NULL,  PRIMARY KEY  (`depid`) )

engine=innodb default charset=utf-8;

insert into dept(`depid`,`deptname`) values (\'01\',\'部门1\'),(\'02\',\'部门2\'),(\'03\',\'部门3\'),(\'04\',\'部门4\');  

drop table if exists "emp";

create table emp (`empid` varchar(50) NOT NULL,   `empname` varchar(50) default NULL,    `depid` varchar(50) default NULL,PRIMARY KEY  (`empid`) ) engine=innodb default charset=utf-8;

insert into emp(`empid`,`empname`,`depid`) values (\'0001\',\'柳梦璃\',\'01\'),(\'0002\',\'韩菱纱\',\'01\'),(\'0003\',\'云天河\',\'02\'),(\'0004\',\'慕容紫英\',\'03\'),(\'0005\',\'玄霄\',NULL),(\'0006\',\'九天玄女\',NULL); 

左连接：select * from dept d left join emp e where d.depid=e.empid;

右连接：select * from dept d right join emp e where d.depid=e.empid;

内链接：select * from dept d innner/union join emp e where d.depid=e.empid;

(内链接可以省略inner/union关键字）