mysql增删改查

Posted 2021-01-21 杨帆，起航！

                                 mysql增删改查

查

show databses;   #查看数据库

select database();  #查看当前数据库

use mysql;    #进入数据库

show create database p1807;  #查看数据库字符集类型

 

desc xxxx;  #查看表类型

select * from students;   #查看表的内容

show create table students;  #查看创建的表

show tables;  #查看所有表

 

select * from students where age in (12,18,34);

select * from students where (age between 18 and 34) order by high desc, age desc;

select sum(age)/count(*) from students; 

select round(avg(age), 2) from students; 

 

计算人数

select count(*) as ‘男性‘ from students where gender=1;

select gender, count(*) from students group by gender; 

select gender, count(*) from students where gender=1 group by gender; 

 

拼接

select concat(id,‘ ‘,name,‘ ‘,age) from students;    

select gender, group_concat(name,age," ",id) from students where gender=1 group by gender;

select gender, group_concat(name), avg(age) from students group by gender having avg(age)>30;

select gender, group_concat(name) from students group by gender having count(*)>4;

 

分页

select * from students limit 2;

select * from students limit 0, 2;

select * from students where gender=2 order by high asc limit 0, 2;

 

 

创建（表，库）

 

create database test05; 创建数据库

create database `p-1807`;  特殊字符用``

create database p1807 charset utf8;默认字符类型为utf8

 

 

--创建students表 (id, name, age, high, gender, cls_id)

create table students (

id int unsigned not null auto_increment primary key,

name varchar(20),

age tinyint unsigned default 0,

high decimal(5,2),

gender enum(‘男‘, ‘女‘, ‘中性‘, ‘保密‘) default ‘保密‘,

cls_id int unsigned

);

 

create table xxxx(id int, name varchar(20));

create table yyyy(id int primary key not null auto_increment, name varchar(30));

 

 

--创建classes表(id, name)

create table classes(

id int unsigned not null auto_increment primary key,

name varchar(20)

);

 

增（添加数据）

insert into yyyy values (1,‘老铁‘); #1表示id往表里面添加数据

insert into yyyy values (0,‘老铁‘); #0表示自增

 

insert into students values(0,‘金星‘,18, 188.88, ‘中性‘, 0);

 

insert into students values (0, ‘小乔‘, 20, ‘女‘, 1, ‘1990-01-01‘)

insert into students values (null, ‘蔡文姬‘, 20, ‘女‘, 1, ‘1990-01-01‘)

insert into students values (default, ‘科比‘, 20, ‘1‘, 1, ‘1990-01-01‘)

insert into students values (default, ‘ 康师傅‘, 20, ‘3‘, 1, ‘1990-01-01‘)

insert into students(name, gender) values (‘大乔‘, 2),(‘貂蝉‘, 2);

 

改（列表名，类型）

                     表名               列名       类型 

alter table students add birthday datetime;  在表里添加列表

 

(modify 和chang 都是修改 用哪个根据个人喜好

修改列表名或类型

alter table students modify birthday date; 

alter table students change birthday birth date default ‘2000-01-01‘;

update students set  age=15 where id=8;  修改某列某行数据

alter database students default character set=uft8; 修改数据库默认字符集类型

 

删（行，列，库）

delete from students where id=15;  删除某行

alter table students drop high;  删除列

drop database test03; 删除数据库

 

-- 逻辑删除

 

-- 用一条字段来表示 这条信息是否已经不能在使用了

-- 给students表添加一个is_delete字段 bit 类型

-- alter table students add is_delete bit default 0;

-- update students set is_delete=1 where id=6;

 

alter table students add is_delete bit default 0;

select * from students where is_delete=0;

 

bit(1)=1,0

bit(2)=(0,0,),(0,1),(1,0),(1,1)

 

-- 数据库的备份与恢复(shell下运行)

备份

mysqldump -uroot -proot --databases testdb > testdb.sql  

mysqldump -uroot -proot --all-databases > all_databases.sql  备份所有库

mysqldump -uroot -proot testdb students > students.sql  备份表

 

恢复

source testdb.sql