通过实例简要介绍case函数的用法。

1.创建测试表:

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;

2.查看相应数据

SQL> select * from students;ID FIRST_NAME           LAST_NAME            MAJOR                          CURRENT_CREDITS GR
---------- -------------------- -------------------- ------------------------------ --------------- --10000 Scott                Smith                Computer Science                            9810001 Margaret             Mason                History                                     8810002 Joanne               Junebug              Computer Science                            7510003 Manish               Murgratroid          Economics                                   66

3.更新语句

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
)
/

4.更新后结果

SQL> select * from students;ID FIRST_NAME           LAST_NAME            MAJOR                          CURRENT_CREDITS GR
---------- -------------------- -------------------- ------------------------------ --------------- --10000 Scott                Smith                Computer Science                            98 a10001 Margaret             Mason                History                                     88 b10002 Joanne               Junebug              Computer Science                            75 c10003 Manish               Murgratroid          Economics                                   66 d