操作数据库的步骤：连接, 打开库, 操作, 关闭退出
MySQL版本：Ver 8.0.20
1、连接数据库（在cmd终端）
mysql -h localhost -u root –p
可能还有-P（端口号，默认3306）
2、sql语法特点：
（1）SQL 语句可以换行, 要以分号结尾
（2）如果提示符为 '>’， 那么需要输入一个回车
（3）命令打错了换行后不能修改, 可以用 c 取消
3、数据库操作
show databases;
create database testdb default charset=utf8;
create database if not exists testdb default charset=utf8mb4;
use testdb;
drop database testdb;
4、数据表操作
show tables;
建表1：
create table score(
name varchar(10),
scroe int
)engine=innodb default charset=utf8;
建表2：
create table if not exists users(
id int unsigned not null primary key auto_increment,
name varchar(4) not null,
age tinyint,
sex enum('男','女')
)engine=innodb default charset=utf8;
查看表结构：
desc score;
查看建表语句：
show create table score;
删除数据表：
drop table testtable;
修改表名：
alter table score rename as stu_score;
更改表中自增的值的起始值：
alter table stu_score auto_increment=100;
查看表状态信息：
show table status from djldb where name='score'G;
修改表引擎：
alter table score engine='myisam';
5、数据操作（增删改查）
插入：
insert into score(name,scroe) values('jacky',82);
insert into score(name,scroe) values('Tom',70),('Jessica',85);
查询：
select * from score;
select name,scroe from score;
select * from score where name='jacky';
select 字段|* from 表名
[where 搜索条件]
[group by 分组字段 [having 分组条件]]
[order by 排序字段 排序规则]
[limit 分页参数]
select * from user where age not between 20 and 30;
select * from user where age between 20 and 30 and sex='女';
select * from user where (age=23 or age=18) and sex='女'; #不加括号，默认先and再or
select * from user where name like '%j%'; # 模糊搜索，name中含有j的
select * from user where name like '__'; # 模糊搜索，name是2个字符的
修改：
update score set scroe=90 where name='jacky';
update score set scroe=100,name='ding' where name='jacky';
update score set scroe=scroe-6 where name='ding';
update stu_score set score=90 where score=70 or score=85;
update stu_score set score=90 where score in(70,85); # 等价于上面那句
删除：
delete from score where name='ding';
delete from stu_score where sid>=100 and sid<=101;
delete from stu_score where sid between 100 and 101; # 等价于上面那句
6、退出数据库
q或者exit 或者 quit
7、修改表结构
添加字段：
alter table user add num int not null;
alter table user add email varchar(20) after age;
alter table user add pid int(11) not null default "1" after age;
alter table user add aaa int first;
删除字段：
alter table user drop aaa;
修改字段（modify不能修改字段名；change可以修改字段名）：
alter table user modify num tinyint not null default 10; #如果之前num列有null值则应该先改变null值为其他值
alter table user change num mm char(5);
8、统计函数（聚合函数）
select max(age),min(age),sum(age),avg(age),count(age) from user;
select max(age) as max_age, min(age) min_age,
sum(age),avg(age) avg_age,
count(age) from user; # 起别名，as可有可无
select count(*) from user;
select count(phone) from user; # null值不计数，空值计数
select classid,count(*) from user group by classid;
select classid,sex,count(*) as num from user group by classid,sex; # 二次分组
# 统计classid=2的男女生人数
select classid,sex,count(*) from user where classid=2 group by sex;
# 统计每个班的平均age，并按照平均age降序排列
select classid,avg(age) from user group by classid order by avg(age) desc;
# 统计女生最多的班级
select classid,sex,count(*) from user where sex='女' group by classid order by count(*) desc limit 1;
# having对分组结果再过滤
select classid,count(*) from user group by classid having count(*)>=4;
9、排序
select * from user order by age desc; # desc降序；默认升序
select * from user order by classid, age desc; #多字段排序
10、提取部分数据（分页）
select * from user limit 2; # 显示2条
select * from user limit 3,2; # 跳过3条，显示2条；
select * from user order by age desc limit 2; # 显示age最大的2条数据
11、其他
md5加密：
select md5('123456');
查看支持的字符集：
show charset;
12、导出（不要连接mysql，直接在cmd中操作）
数据库导出：
mysqldump -u root -p djldb > C:UsersloriDesktopmysql_codedjldb.sql
数据表导出：
mysqldump -u root -p djldb user > C:UsersloriDesktopmysql_codedjldb_user.sql
13、导入（事先已建好djldb2库；不要连接mysql，直接在cmd中操作）
mysql -u root -p djldb2 < C:UsersloriDesktop mysql_code djldb_user.sql
mysql -u root -p djldb2 < C:UsersloriDesktop mysql_code djldb.sql
14、用户权限管理
查看现有用户：
use mysql;
select host,user from user;
新建用户，并授权相应权限：
# 创建zhangsan 用户，登录地址任意，密码123456
create user 'zhangsan'@'%' identified by '123456';
# 给zhangsan授权djldb2库的select,insert权限
grant select,insert on djldb2.* to 'zhangsan'@'%';
新建用户lisi，并授权djldb2的所有权限：
create user 'lisi'@'%' identified by '123456';
grant all on djldb2.* to 'lisi'@'%';
删除用户：
drop user 'lisi'@'%';
15、根据计算结果进行查询和排序
# djltest是表名，Eng和Math是列名，根据Eng和Math计算出了新的数据totle，根据此数据进行排序
select *, Eng+Math as totle from djltest order by totle desc;
select id, Eng+Math as totle from djltest order by totle desc;