前言

DECODE()函数，它将输入数值与函数中的参数列表相比较，根据输入值返回一个对应值。函数的参数列表是由若干数值及其对应结果值组成的若干序偶形式。当然，如果未能与任何一个实参序偶匹配成功，则函数也有默认的返回值。

语法结构如下：

decode (expression, search_1, result_1)
decode (expression, search_1, result_1, search_2, result_2)
decode (expression, search_1, result_1, search_2, result_2, ...., search_n, result_n)

decode (expression, search_1, result_1, default)
decode (expression, search_1, result_1, search_2, result_2, default)
decode (expression, search_1, result_1, search_2, result_2, ...., search_n, result_n, default)

实例使用

1. 创建一个学生成绩表。栏位有： 学生名， 科目名和分数

create table student_score(
   name varchar2(30),
   subject varchar2(20),
   score number(4,1)
);

insert into student_score (name,subject,score)values(‘zhang san‘,‘Chinese‘,90);
insert into student_score (name,subject,score)values(‘zhang san‘,‘Mathematics‘,80);
insert into student_score (name,subject,score)values(‘zhang san‘,‘English‘,79);

测试一：

select name,subject,decode(subject, ‘Chinese‘,score,0) from student_score;

如果是中文课程的话， 显示分数， 其他课程分数为零。

这条SQL 看上去使用意义不大。

测试二：

select name,sum(decode(subject, ‘Chinese‘,score,0)) as CHINESE from student_score group by name;

统计中文课程的分数。看上去有点意义。

总体看来, decode 的使用看上去和case when 有点类似。如果只是用作以上两种状况，看上去意义不大。

select name,sum(decode(subject, ‘Chinese‘,score,0)) as CHINESE from student_score group by name;
select name,score as CHINESE from student_score;

行转列-有意义的使用

往以上table 再插入一些其他学生的成绩：

insert into student_score (name,subject,score)values(‘li shi‘,‘Chinese‘,96);
insert into student_score (name,subject,score)values(‘li shi‘,‘Mathematics‘,86);
insert into student_score (name,subject,score)values(‘li shi‘,‘English‘,76);

insert into student_score (name,subject,score)values(‘wang wu‘,‘Chinese‘,92);
insert into student_score (name,subject,score)values(‘wang wu‘,‘Mathematics‘,82);
insert into student_score (name,subject,score)values(‘wang wu‘,‘English‘,72);

select name,
sum(decode(subject, ‘Chinese‘, nvl(score, 0), 0)) "Chinese",
sum(decode(subject, ‘Mathematics‘, nvl(score, 0), 0)) "Mathematics",
sum(decode(subject, ‘English‘, nvl(score, 0), 0)) "English"
from student_score
group by name;

返回：

将行的数据转化为列， 是不是很有意义了。

使用case then 也可以达到相同的效果。

select name,
sum(case when subject=‘Chinese‘
              then nvl(score,0)
         else 0
    end) "Chinese",
sum(case when subject=‘Mathematics‘
              then nvl(score,0)
         else 0
    end) "Mathematics",
sum(case when subject=‘English‘
              then nvl(score,0)
         else 0
    end) "English"
from student_score
group by name;

[Oracle] decode 函数及其用法