DROP SEQUENCE student_sequence;
CREATE SEQUENCE student_sequence START WITH 10000 INCREMENT BY 1;

DROP TABLE students;
CREATE TABLE students (
id NUMBER(5) PRIMARY KEY,
first_name VARCHAR2(20),
last_name VARCHAR2(20),
major VARCHAR2(30),
current_credits NUMBER(3),
grade varchar2(2));

INSERT INTO students (id,first_name,last_name,major,current_credits,grade)
VALUES (student_sequence.NEXTVAL,'Scott','Smith','Computer Science',98,null);

INSERT INTO students (id,'Margaret','Mason','History',88,'Joanne','Junebug',75,'Manish','Murgratroid','Economics',66,null);

commit;

sql> select * from students;

ID FIRST_NAME LAST_NAME MAJOR CURRENT_CREDITS GR
---------- -------------------- -------------------- ------------------------------ --------------- --
10000 Scott Smith Computer Science 98
10001 Margaret Mason History 88
10002 Joanne Junebug Computer Science 75
10003 Manish Murgratroid Economics 66

update students
set grade = (
select grade from
(
select id,
case when current_credits > 90 then 'a'
when current_credits > 80 then 'b'
when current_credits > 70 then 'c'
else 'd' end grade
from students
) a
where a.id = students.id
)
/

sql> select * from students;

ID FIRST_NAME LAST_NAME MAJOR CURRENT_CREDITS GR
---------- -------------------- -------------------- ------------------------------ --------------- --
10000 Scott Smith Computer Science 98 a
10001 Margaret Mason History 88 b
10002 Joanne Junebug Computer Science 75 c
10003 Manish Murgratroid Economics 66 d