显示有哪些数据库：show databases;
创建数据库：create database database1;
删除数据库：drop database database1;
使用数据库：use database1;
查看表：show tables;
新建表：create table table1(
id int(10) not null primary key auto increment ,
a varchar(20),
b int(10),
c varchar(10) default '男',
d int(10)
);
查看表结构:desc student;
删除表:drop table student;
删除主键:alter table table1 drop primary key;
增加主键:alter table table1 add primary key(id);
删除字段:alter table table1 drop classid;
增加字段:alter table table1 add classid int(10);
增加记录:insert into table1(a,b,c,d) values('哈哈',1,'你好',2);
删除：delete from table1 where b = 1;
修改:update table1 set a = '哈哈哈' where b=1;
通配符 %:多个任意的字符 _:一个字符
update table1 set b = 2 where name like '%i%';
查询:
select * from table1;
select id,name from table1;
select * from table1 where b like '%i%';
排序 默认升序asc 降序desc
select * from table1 order by a asc;
select * from table1 order by a desc;
select * from table1 order by a desc,b asc;
组函数 min max count avg
select count(*) from table1;
select min(a) from table1;
select avg(a) from table1;
select max(a) from table1;
分组:select classid,avg(b) from table1 group by classid having avg(b)>5;
字句作为查询的条件:select * from table1 where age = (select min(b) from table1);
