MySQL——索引，连表……

Posted 2021-12-03 mr-feng

0. 唯一索引

create table t1(
id int ....,
num int,
xx int,
unique 唯一索引名称 (列名,列名),
constraint ....
)
#
1 1 1
2 1 2
PS:
唯一：
约束不能重复（可以为空）
PS: 主键不能重复（不能为空）
加速查找

1. 外键的变种

a. 用户表和部门表

用户：
1 alex 1
2 root 1
3 egon 2
4 laoyao 3

部门:
1 服务
2 保安
3 公关
===》 一对多
b. 用户表和博客表
用户表：
1 alex
2 root
3 egon
4 laoyao
博客表：
FK() + 唯一
1 /yuanchenqi/ 4
2 /alex3714/ 1
3 /asdfasdf/ 3
4 /ffffffff/ 2

===> 一对一

create table userinfo1(
id int auto_increment primary key,
name char(10),
gender char(10),
email varchar(64)
)engine=innodb default charset=utf8;

create table admin(
id int not null auto_increment primary key,
username varchar(64) not null,
password VARCHAR(64) not null,
user_id int not null,
unique uq_u1 (user_id),
CONSTRAINT fk_admin_u1 FOREIGN key (user_id) REFERENCES userinfo1(id)
)engine=innodb default charset=utf8;

c. 用户表（百合网） 相亲记录表

示例1：
用户表
相亲表

示例2：
用户表
主机表
用户主机关系表
===》多对多

create table userinfo2(
id int auto_increment primary key,
name char(10),
gender char(10),
email varchar(64)
)engine=innodb default charset=utf8;

create table host(
id int auto_increment primary key,
hostname char(64)
)engine=innodb default charset=utf8;

create table user2host(
id int auto_increment primary key,
userid int not null,
hostid int not null,
unique uq_user_host (userid,hostid),
CONSTRAINT fk_u2h_user FOREIGN key (userid) REFERENCES userinfo2(id),
CONSTRAINT fk_u2h_host FOREIGN key (hostid) REFERENCES host(id)
)engine=innodb default charset=utf8;


2. SQL语句数据行操作补充
create table tb12(
id int auto_increment primary key,
name varchar(32),
age int
)engine=innodb default charset=utf8;

增
insert into tb11(name,age) values(‘alex‘,12);

insert into tb11(name,age) values(‘alex‘,12),(‘root‘,18);

insert into tb12(name,age) select name,age from tb11;
删
delete from tb12;
delete from tb12 where id !=2
delete from tb12 where id =2
delete from tb12 where id > 2
delete from tb12 where id >=2
delete from tb12 where id >=2 or name=‘alex‘

改
update tb12 set name=‘alex‘ where id>12 and name=‘xx‘
update tb12 set name=‘alex‘,age=19 where id>12 and name=‘xx‘
查

select * from tb12;

select id,name from tb12;

select id,name from tb12 where id > 10 or name =‘xxx‘;

select id,name as cname from tb12 where id > 10 or name =‘xxx‘;

select name,age,11 from tb12;

其他：
select * from tb12 where id != 1
select * from tb12 where id in (1,5,12);
select * from tb12 where id not in (1,5,12);
select * from tb12 where id in (select id from tb11)
select * from tb12 where id between 5 and 12;


通配符：

select * from tb12 where name like "a%"　　%表示任意
select * from tb12 where name like "a_"　　_表示一个


分页：

select * from tb12 limit 10;　　　　前十个

select * from tb12 limit 0,10;　　从0向后取10
select * from tb12 limit 10,10;　　从10 向后取10
select * from tb12 limit 20,10;　　

select * from tb12 limit 10 offset 20;
从第20行开始读取，读取10行；

结合Python分页：
# page = input(‘请输入要查看的页码‘)
# page = int(page)
# (page-1) * 10
# select * from tb12 limit 0,10;  1 页
# select * from tb12 limit 10,10; 2 页


排序：
select * from tb12 order by id desc; 大到小
select * from tb12 order by id asc; 小到大
select * from tb12 order by age desc,id desc;　　先按age排序，如果age一样按id排序

取后10条数据
select * from tb12 order by id desc limit 10;

分组：

select count(id),max(id),part_id from userinfo5 group by part_id;

count　
max　
min
sum
avg

**** 如果对于聚合函数结果进行二次筛选时？必须使用having ****
select count(id),part_id from userinfo5 group by part_id having count(id) > 1;

select count(id),part_id from userinfo5 where id > 0 group by part_id having count(id) > 1;


连表操作：

select * from userinfo5,department5

select * from userinfo5,department5 where userinfo5.part_id = department5.id

select * from userinfo5 left join department5 on userinfo5.part_id = department5.id
select * from department5 left join userinfo5 on userinfo5.part_id = department5.id
# userinfo5左边全部显示


# select * from userinfo5 right join department5 on userinfo5.part_id = department5.id
# department5右边全部显示



select * from userinfo5 innder join department5 on userinfo5.part_id = department5.id
将出现null时一行隐藏






select * from
department5
left join userinfo5 on userinfo5.part_id = department5.id
left join userinfo6 on userinfo5.part_id = department5.id


select
score.sid,
student.sid
from
score

left join student on score.student_id = student.sid

left join course on score.course_id = course.cid

left join class on student.class_id = class.cid

left join teacher on course.teacher_id=teacher.tid