创建表，班级，属性年级，班级号

create table  class(
    id number primary key ,
    grade varchar2(6),
    class_no number
);

删除表

drop table class;

删除一条

delete from mytable where id= 1 ;

插入全部和部分

insert into class values(1,'高一',01);

insert into class(id,grade) values(2,'高二');

查询空字段

select *from class where class_no is null ;

左外连接

select * from student s 
left join class c on s.class_id=c.id
where ...and...

分组

1.常用的分组函数：AVG平均值，SUM求和，MIN最小值，MAX最大值，COUNT个数，WM_CONCAT字符串的拼接等。
2.group by 分组查询，select子句后的任一非聚合函数字段都应来源于group by 分组语句后，否则语法会编译不通过。
3.order by是具体条件

select sex,MAX(age) from student group by sex;

查多个分组字段

select name,age from student
group by age,name 
order by age;

where+group by+having

SELECT SNAME,SAGE FROM STUDENT
 where SSEX='男'
 GROUP BY SNAME,SAGE 
HAVING SAGE>'21'