sql select update delete insert

conn scott/root;
create table student (id number(3), name varchar2(10), sex char(2), sno number(3));
alter table student add(sal number(7,2), birthday date);
alter table student modify(name varchar2(10));
alter table student drop column sal;
rename student to stu;
rename stu to student;
create table classes(class_id number(3), class_name varchar2(10));
desc student;
insert into student values(001, 'Tom', '男', 1, '22-4月-82', 100.50);
insert into student values(003, 'Mary', '女', 3, '24-5月-1988', 3000);
alter session set nls_date_format = 'yyyy-mm-dd';
insert into student values(004, '张三', '男', 4, '1997-7-1', 800);
insert into student(id, name, sex) values(5, 'Test', '女');
insert into student(id, name, sex, birthday) values(6, 'Test2', '男', null);
select * from student where birthday is not null;
select * from student where birthday is null;
update student set name = 'Test_1' where name = 'Test';
update student set name = 'Test_2', birthday = '2000-9-9' where id = 6;
delete student where birthday = '1997-7-1';
delete from student where id = 5;