表管理和索引，外键作用

表管理和索引，外键作用：

创建数据库
create schema if not exists students character set 'gbk' collate 'gbk_chinese_ci';

cd students
ls
file db.opt
cat db.opt
default-character-set=gbk
default-collation=gbk_chinese_ci

修改数据库
help alter database;
help drop databases;

help create table;

创建表的3种方法：
1）直接定义一张空表；
2)从查询结果中创建新表
3）以其他表为模板创建一个空表 like关键字

单字段：
  primary key 主键
  unique key 唯一键

单或多字段：
  primary key (col,...)
  unique key (col,...)
  index (col,...)


create table if not exists tb_name (col_name col_definition,constraint)

create table tb1 (id int unsigned not null auto_increment primary key,name char(20) not null,age tinyint not null)


id字段设置为主键
create table tb2 (id int unsigned not null auto_increment ,name char(20) not null,age tinyint not null,primary key(id))


主键，唯一键，索引字段
create table tb3 (id int unsigned not null auto_increment ,name char(20) not null,age tinyint not null,primary key(id),unique key(name),index(age)) engine = engine_name

主键 唯一键 索引键的却别：
键也称作约束，可用作索引，属于是特殊索引（有特殊限定）：B+Tree

create index

create table courses(cid tinyint unsigned not null auto_increment primary key,couse varchar(50) not null) engine = innodb;

show table status like 'courses';
show table status like 'courses'G

drop table courses;

insert into courses(couse) values('python'),('c++'),('mysql');
select * from courses;
show indexes from courses;


显示指定表的索引
show indexes from tb_name;

查询结果创建新表
create table testcourses select * from courses where cid <=2;

show tables;
select * from testcourses;

显示表结构
desc courses;
desc testcourses;

以其他表为模板创建一张空表
create table test like courses;
desc test;

修改表结构：alter table
  help alter table
  增删改 字段
  增删改 索引
  修改表属性

show indexes from test;

alter table test add unique key (couse);  添加唯一键索引

alter table test change couse course varchar(50) not null;  修改字段名称

alter table test add startdate date default '2020-01-01'; 增加字段

help alter table

alter table test rename as|to courses2;  修改表名称

rename table courses2 to test;

删除表
help drop table

drop table if exists tb_name;

create table student (sid int unsigned not null auto_increment primary key,name varchar(30),cid int not null);

insert into student (name,cid) values ('wqd',2),('zqd',1);

select * from student;
select * from courses;
select name,couse from student,courses where student.cid=courses.cid;

insert into student (name,cid) values('chenchen',5);
delete from student where cid =5;

添加外键
作用：学生表的cid和课程表的cid 引用外键约束，学生表里插入不存在的课程时 会报错
alter table student modify cid tinyint unsigned not null;

alter table student add foreign key foreign_cid(cid) references courses(cid);

show indexes from student;


修改表的引擎：
alter table courses engine = innodb;

alter table courses engine = mysiam;

insert into student (name,cid) values ('chenchen',3);

delete from courses where cid=3;   会报错

只有innodb引擎才支持外键
外键约束能够防止 表 被误删

索引：创建 和 修改
help create index ;
help drop index;

create index index_name on tb_name;
drop index index_name on tb_name;

create index name_on_student on student (name) using btree;
create index name_on_student on student (name(5) desc) using btree;  长度5字符 降序

drop index name_on_student on student;