SQL——第一步

SQL的数据定义语言（DDL）部分主要面向表和数据库：

创建：create database/table

修改：alter database/table 

删除：drop database/table

/* alter即键盘上的Alt(er)键的语义 */

SQL的数据操作语言（DML）部分主要面向表中的记录，就是四个功能：

增：insert into

删: delete from where

改: update set where

查: select from where

现使用MySql实践SQL。

1,进入MySql创建数据库，并切换到所创建的数据库：

show databases;
create database test;
--Query Ok, 1 row affected (0.00 sec)--
/* 思考：为什么是"1 row affected",row在哪儿 */

show databases;
use test;
select database();

2，创建一个测试用表：

show tables;
create table students
(
id int(2),    # 2指示int的最大位数为2
name varchar(8) # 8指示varchar的最大长度为8
);
show tables;
desc students;

3, 修改这个表：

alter table students add column *** int(1);
desc students;
alter table students drop column ***;
desc students;
alter table students change id id(4) not null primary key;
desc students;

4, 现在我们便可以使用数据操作语言（DML）来进行记录级的操作:

insert into students values (1,"alice") --Query OK--
insert into students values (1,"bob")     --ERROR: Duplicate entry '1' for key 1--
insert into students values (2,"bob")    --Query OK--
insert into students values (3,"cindy") --Query OK--
select * from students;

delete from students where name = "cindy";
select * from students;

update students set name = "bobbod" where id = 2;
select * from students;

select name from students where id = 1;

5,完成基础学习，清理实验平台：

desc students;
select * from students;

show tables;
drop table students;
show tables;

show databases;
drop database test;
show databases;

select database();