case when ... then .. when .. then ... end
这种表达式，是sql的不同条件分支结果。
举例如下，
1、创建数据表，create table test_student(stu_id number, class_id number);
2、插入测试数据，
insert into test_student values(1,1001);
insert into test_student values(2,1001);
insert into test_student values(3,1002);
insert into test_student values(4,1003);
insert into test_student values(5,1003);
insert into test_student values(6,1003);
3、查询表中数据，select * from test_student ,
4、编写case when 表达式，将class_id翻译为中文名称，sql如下，
select t.*,
case
when class_id = 1001 then
'语文'
when class_id = 1002 then
'数学'
when class_id = 1003 then
'英语'
end as class_name
from test_student t;
