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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_scoregroup 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_scoregroup by name;

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