mysql 增删改查以及连接

创建表：
CREATE TABLE teacher(
id CHAR(5) PRIMARY KEY,
name varchar(20) NOT NULL,
gender enum('man','women') NOT NULL,
poisition enum('助教‘,'讲师','客服'
);
插入
INSERT INTO（表民）VALUES（内容）
普通插入

INSERT INTO `teachers` VALUES('12','12','MAN','讲师');

指定插入

insert into `teachers` ('id','gender','name') values('0511','man','hello')

插入多行数据

insert into `teachers` 
          values ('111','nihao','man'),
                      ('222','www','women'),
                      ('333','nnn','man');

修改

update  `teachers` set `position` = '讲师’ where `id`= '05110'

查询

select  *  from `teachers` where 1

*   代表列
where 后面代表行

select 'name','gender' from `teachers` where 1;

指定行查询

select `name` from `teachers`  where  `position`= '讲师';

select `name` from `teachers`  where  `position`= '讲师'  or `position` = '教授';

select `name` from `teachers`  where  (`position`= '讲师'  or `position` = '教授')  and `gender` = `man`;
优先级问题

排序：
select `name` from `teachers`  where  (`position`= '讲师'  or `position` = '教授')  and `gender` = `man` order by `id` desc(降序）

多条件排序：
select `name` from `teachers`  where  (`position`= '讲师'  or `position` = '教授')  and `gender` = `man` order by `id` desc，name;

分组查询：
GROUP BY 通常不和order by 一起使用，group by通常是配合聚合函数一起使用。

select `name` from `teachers`  where  1 group by `gender`

select count(*) from `teachers`  where  1 group by `gender`; 计数

select `gender`,count(*) from `teachers`  where  1 group by `gender`

别名
select `gender` as `GENDER`,count(*)  as `num` from `teachers`  where  1 group by `gender`

删除

delete from `teacher` where 1 全删！！跑路吧。。

delete from `teacher` where `name` = 'caoliang'

在删除之前一定要确认where ，同时，需要在相同的where条件先select一次。！！

连接：可以解决80%的问题，还有一种是子查询

1.内连接
第一张表的每一行，和第二张表的每一列都连接一次。

Paste_Image.png

select a.id,a.name from 'class' as a 
inner join 'student' as b 
on a.id= b.id 
where b.id=9 

2.外连接
主要有左连接和右连接，在写语句时，写在前面的表叫左表，后面的叫右表。left join 是保持左边不变，right是保持右边不变。

select a.id,a.name from 'class' as a 
left join 'student' as b 
on a.id= b.id