1. 数据表：

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;

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2. CASE使用：

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
)

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注：此文章转自http://www.eygle.com/archives/2005/10/oracle_howto_user_case_function.html

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附加：

1、以CASE开头，以END结尾

　　2、分支中WHEN 后跟条件，THEN为显示结果

　　3、ELSE 为除此之外的默认情况，类似于高级语言程序中switch case的default，可以不加

　　4、END 后跟别名

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