create table instructor(
	ID     char(5),
    name   varchar(20) not null,
    dept_name varchar(20),
    salary numeric(8,2),
    primary key(ID),
    foreign key(dept_name) references department,
    check(salary>0);
)

insert into instructor values(080040, 'Jack', 'Com.Sci', 8000)

delete from isntructor

drop table r

alter table r add A D

alter table r drop A

select distinct dept_name from instructor # 去重

select all dept_name from instructor # 不去重

select T.name as instructor_name, course_id 
from isntructor as T, instructor as S
where T.salary > S.salary 
	and S.dept_name = 'Comp.Sci'
	and salary between 9000 and 10000
order by name desc # asc上升

select dept_name, avg(salary) as avg_salary
from instructor
group by dept_name