Mysql一些操作

操作文件夹（库）

增
       create database db1 charset utf8;
查
      show create database db1;
      show databases;
改
      alter database db1 charset gbk;
删
      drop database db1;

操作文件（表）

切换文件夹：use db1;
查看当前所在文件夹：select database();
增
         create table t1(id int,name char);
查
         show create table t1;
         show tables;
         desc t1;
改
         alter table t1 modify name char(6);
         alter table t1 change name NAME char(7);
删
         drop table t1;

操作文件内容（记录）

增
        insert t1(id,name) values(1,'egon1'),(2,'egon2'),(3,'egon3');
改
       update db1.t1 set name='SB';
       update db1.t1 set name='ALEX' where id=2;
删
       delete from t1;
       delete from t1 where id=2;
       
       truncate t20; #应该用它来清空表

主键 primary key

unique key
单列唯一
    #方式一
    create table department(
        id int unique,
        name char(10) unique
    );

primary key
约束：not null unique
存储引擎（innodb）:对于innodb存储引擎来说，一张表内必须有一个主键

# 单列主键
create table t17(
    id int primary key,
    name char(16)
);

# 复合主键
create table t19(
    ip char(15),
    port int,
    primary key(ip,port)
);

约束：not null unique
存储引擎（innodb）:对于innodb存储引擎来说，一张表内必须有一个主键

单表查询

select distinct 字段1,字段2,字段3 from 库.表
          where 条件
          group by 分组条件
          having 过滤
          order by 排序字段
          limit n;

例子：

select distinct post,count(id) as emp_count from employee
	where salary > 1000
	group by post
	having count(id) > 1
	order by emp_count desc

循序

def select():
      f=from('db1','t1')
      lines=where('id>3',f)
      group_res=group(lines)
      having_res=having(group_res)
      distinct_res=distinct(having_res)
      order_res=order(distinct_res)
      res=limit(order_res)
      print(res)
      return res

#where

select name,post,salary from employee where post='teacher' and salary > 8000;


select name,salary from employee where salary between 20000 and 30000;


select * from employee where age in (73,81,28);


select * from employee where post_comment is Null;
select * from employee where post_comment is not Null;


select * from employee where name like "jin%";
select * from employee where name like "jin___";

#正则表达式
select * from employee where name like 'jin%';
select * from employee where name regexp '^jin';
select * from employee where name regexp '^jin.*(g|n)$';

表关系

两张表之间的关系：
    1、多对一
        出版社    书（foreign key(press_id) references press(id)）
    2、多对多
        作者       书
        egon: 
            九阳神功
            九阴真经
        alex: 
            九阳神功
            葵花宝典
        yuanhao:
            独孤九剑
            降龙十巴掌
            葵花宝典
        wpq:
            九阳神功
            
        insert into author2book(author_id,book_id) values
        (1,1),
        (1,2),
        (2,1),
        (2,6);
            
        
    3、一对一
        customer表       student表

多表查询

内连接：只取两张表的共同部分
select * from employee inner join department on employee.dep_id = department.id ;

左连接:在内连接的基础上保留左表的记录
select * from employee left join department on employee.dep_id = department.id ;

右连接：在内连接的基础上保留右表的记录
select * from employee right join department on employee.dep_id = department.id ;

全外连接：在内连接的基础上保留左右两表没有对应关系的记录
select * from employee full join department on employee.dep_id = department.id ;


select * from employee left join department on employee.dep_id = department.id
union
select * from employee right join department on employee.dep_id = department.id ;