#yyds干货盘点# 15 MySQL

一、连接

我们现在进行多个表一起查询，也就是我们说的连接查询。连接分为以下几种。

1.内连接

select * from tbl_students 
inner join tbl_classes 
on tbl_students.cls_id = tbl_classes.id;

简写

select * from tbl_students as s 
inner join tbl_classes as c on s.cls_id = c.id

只显示学生的所有信息，班级只显示名字

select s.*,c.name from tbl_students as s 
inner join tbl_classes as c on s.cls_id = c.id;

根据年龄倒序排列

select s.*,c.name 
from tbl_students as s 
inner join tbl_classes as c on s.cls_id = c.id 
order by s.age desc;

2.左连接

select s.*,c.name from tbl_students as s left join tbl_classes as c on s.cls_id = c.id;

查询没有班级的学生

select s.*,c.name from tbl_students as s left join tbl_classes as c on s.cls_id = c.id having c.name is null;

3.右连接

insert into tbl_classes values (0,"精英10班");

insert into tbl_classes values (0,"精英11班");

insert into tbl_classes values (0,"精英12班");

select s.*,c.name 
from tbl_students as s 
right join tbl_classes as c on s.cls_id = c.id;