MySQL(蠕虫复制、limit-truncate-select、别名-数据源、where子句、group by子句)

mysql.exe -h localhost -P 3306 -u root -p

use mydb;     ——     进入数据库

show tables;    ——    查看所有表

查看:show index from 表名\G

desc:查看表结构

select * from 表名:查询所有数据

表创建的高级操作

从已有表创建新表(复制表结构):create table 表名 like 数据库.表名;

蠕虫复制:

先查出数据,然后将查出的数据新增一遍

        insert into 表名[(字段列表)] select 字段列表/* from 数据表名;

蠕虫复制的意义

从已有表拷贝数据到新表中

可以迅速地让表中的数据膨胀到一定的数量级,用来测试表的压力以及效率

-- 复制创建表

create table my_copy like my_class;

-- 蠕虫复制

insert into my_copy selsct * from my_class;

-- 删除主键

alter table my_copy drop primary key;

insert into my_copy selsct * from my_copy;

-- 更新部分1811变成1911

update my_copy set name='Python1911' where name='Python1811' limit 3;

-- 删除数据:限制记录数为5

delete from my_copy where name='Python1903' limit 5;

-- 给my_student表增加主键

alter table my_student modify id int primary key auto_increment;

-- 清空表:重置自增长

truncate my_student;

-- select 选项

select * from my_copy;

select all * from my_copy;

-- 去重

select distinct * from my_copy;

-- 向学生表插入数据

insert into my_student

values (null,'bc20200001','charry','女'),

(null,'bc20200002','marry','女'),

(null,'bc20200003','harry','女'),

(null,'bc20200004','karry','男'),

(null,'bc20200005','barry','男');

-- 字段别名

select id, number as 学号, name as 姓名,

sex 性别 from my_student;

-- 多表数据源

select * from my_student,my_class;

-- 子查询

select * from (select * from my_student) as s;

-- 增加age年龄和height身高字段

alter table my_student add age tinyint unsigend;

alter table my_student add height tinyint unsigend;

-- 增加字段值:rend取得一个0~1之间的随机数,floor向下取整

update my_student set age=floor(rand() *20+20),height=floor(rand() *20+170);

-- 找学生id为1、3、5的学生

select * from my_student where id=1 || id=3 || id=5;-- 逻辑判断

-- 找身高在185~190之间的学生

select * from my_student where height>=185 and height<=190;

select * from my_student where height between 185 and 190;

select * from my_student where height between 190 and 185;

select * from my_student where 1;  -- 所有条件都满足

-- 根据性别分组

select * from my_student group by sex;

-- 分组统计:身高高矮、平均年龄、总年龄

select sex,count(*),max(height),min(height),avg(age),sum(age) from my_student group by sex;

-- 修改id为4的记录,把年龄置位NULL

update my_student set age=null where id=4;

select sex,count(*),max(height),min(height),avg(age),sum(age) from my_student group by sex;

-- 修改id为1的记录,把性别置位女

update my_student set sex='女' where id=1;

-- nan nv

-- 倒序

select sex,count(*), count(age),max(height),min(height),avg(age),sum(age) from my_student group by sex desc;

-- 删除班级表主键

alter table my_class drop primary key;

-- 给班级表增加主键id

alter table my_class add id int primary key auto_increment;

-- 给学生表增加一个班级表的id

alter table my_student add c_id int;

update my_student set c_id=ceil(rand() *4);

insert into my_student values(6,'bc20200006','小芳','女',18,160,2;)

-- 多字段分组,先班级,后男女

select c_id,sex,count(*) from my_student group by c_id,sex; -- 多字段排序

select c_id,sex,count(*),group_concat(name) from my_student group by c_id,sex;

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