1.DDL

1.创建表

 create table if not exists t_student(id integer auto_increment primary key ,name 
 varchar(3) , age int);

drop table if exists t_student;

alter table t_student add address varchar(4);
alter table t_student modify address varchar(6) not null;

tip：
简单约束：
name varchar(3) not null
age int default 66
主键约束：
id integer auto_increment primary key

2. DML

1.插入数据

insert into  t_student(name,age,address) values ('xiaoM',12,'guangzhou');

2.修改数据

update t_student set name='xiaoHuang' where id=1;

3.删除数据

delete from t_student where id= 1 ;

tip：
条件约束
delete from t_student where id is 1 or name = 'xiaoM';

查询语句
1.查询所有字段信息

select * from t_student;

2.查询部分字段信息

select name,age from t_student;

3.查询符合条件下的部分字段信息

select name,age from t_student where id=2;

4.查询数据个数

select count(*) from t_student;

5.查询age不为空值的个数

select count(age) from t_student;

6.查询age平均值

select avg(age) from t_student;

7.查询age总和

select sum(age) from t_student;

8.查询age最大值

select max(age) from t_student;

9.查询age最小值

select min(age) from t_student;

10.查询按分数升序，按年龄降序

select *from t_student order by score asc, age desc;