作者前言

 欢迎小可爱们前来借鉴我的gtiee秦老大大 (qin-laoda) - Gitee.com

__________________________________________________________

目录

join连接

•  内连接

• 左连接

• 右连接

• 外连接

• 其他连接

______________________________________________________________

作者小插曲

        最近学校课程很多,无法及时更新,还望各位小可爱多多包含,你的三连是我的动力

插播小知识

1.当我们拼接的时候,有些表的字段数不够就用null补上

•  内连接

select  
*
from 
(select * from new_employees limit 10)as ne
join 
(select * from employees limit 10)as em
on ne.emp_no=em.emp_no;

• 左连接

select  
*
from 
(select * from new_employees limit 20)as ne
left join 
(select * from employees limit 10)as em
on ne.emp_no=em.emp_no;

• 右连接

select  
*
from 
(select * from new_employees limit 10)as ne
right join 
(select * from employees limit 20)as em
on ne.emp_no=em.emp_no;

• 外连接

select  
*
from 
(select * from new_employees limit 20)as ne
left join 
(select * from employees limit 10)as em
on ne.emp_no=em.emp_no
union 
select  
*
from 
(select * from new_employees limit 10)as ne
right join 
(select * from employees limit 20)as em
on ne.emp_no=em.emp_no;

这里的代码我就不使用图片表示了,在上一篇的关于jion 的这些讲了很清楚了,现在我们来接着下面来

其他连接

其他连接一:

 可以看出在这个连接是取左连接的匹配null部分,我们的思路就是左连接后再筛选出来,

select  
ne.emp_no,

ne.`age`,
ne.`gender`,
em.`birth_date`,
em.`first_name`,
em.`last_name`,
em.`gender`,
em.`hire_date`
from 
(select * from new_employees limit 20)as ne
left join 
(select * from employees limit 10)as em
on ne.emp_no=em.emp_no
where em.`emp_no` is null;

结果:

 同理

其他连接二:

select  
ne.emp_no,
ne.`age`,
ne.`gender`,
em.`birth_date`,
em.`first_name`,
em.`last_name`,
em.`gender`,
em.`hire_date`
from 
(select * from new_employees limit 10)as ne
right join 
(select * from employees limit 20)as em
on ne.emp_no=em.emp_no
where ne.`emp_no` is null;

 其他连接3:

 思路就是外连接取含有null的部分，先左连接表格，然后筛选出null的部分，再右连接，筛选出null的部分，然后利用union  distinct拼接表格，

select  
*
from 
(select * from new_employees limit 20)as ne
left join 
(select * from employees limit 10)as em
on ne.emp_no=em.emp_no
where em.emp_no is null
union 
select  
*
from 
(select * from new_employees limit 10)as ne
right join 
(select * from employees limit 20)as em
on ne.emp_no=em.emp_no
where ne.emp_no is null;

注意一下,左连接和右连接都是要有条件筛选的

自关联

把竖表变横表

简单的说就是自己跟自己关联

这张表的内容

这张表自连接

select * from 
city as a
join
city  as b
on a.`id` = b. pid;

子查询

select 
*
from
(select * from city limit 10) as a;

-- 标量子查询
-- 查询大于等于公司平均年龄的员工
select * from table
where age >= (select avg(age) from table);
-- 列级子查询
select * from table1
where id in (select id from table2);
-- 行级子查询
select * from table
where (height,age) = (select max(height),max(age) from table);

 select 
 *
 from new_employees as b
join 
 (
    select avg(age) as av from new_employees 
 )as a 
 on b.age>= a.av
 limit 10;