如何使用Oracle case函数

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, first_name, last_name, major, current_credits,grade)
  VALUES (student_sequence.NEXTVAL, 'Margaret', 'Mason', 'History', 88,null);

INSERT INTO students (id, first_name, last_name, major, current_credits,grade)
  VALUES (student_sequence.NEXTVAL, 'Joanne', 'Junebug', 'Computer Science', 75,null);

INSERT INTO students (id, first_name, last_name, major, current_credits,grade)
  VALUES (student_sequence.NEXTVAL, '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