create view v$_dept_avgsal
as
select deptno, avg_sal, grade from (select deptno, avg(sal) avg_sal from emp group by deptno) t, salgrade s where t.avg_sal between s.losal and s.hisal

select dname, grade
  from v$_dept_avgsal t1,
       dept
 where t1.deptno = dept.deptno
   and t1.grade =
       (select min(grade)
          from v$_dept_avgsal);

create view v$_aaa
as
select * from aaa;

create table t_user(
id number,
username varchar2(30),
password varchar2(30)
);

create table emp_e1 as select * from emp where 1=1;

alter table emp_e1 modify(ename varchar2(10));
insert into emp_e1(empno,ename) values(9898,'cai10cai10cai');

alter table emp_e1 add(phone varchar2(11));

alter table emp_e1 drop column phone;

select * from emp_e1