Oracle數據庫中decode函數的使用

一、decode函數的使用方法：

DECODE()函數，它將輸入數值與函數中的參數列表相比較，根據輸入值返回一個對應值。函數的參數列表是由若干數值及其對應結果值組成的若干序偶形式。當然，如果未能與任何一個實參序偶匹配成功，則函數也有默認的返回值。
語法結構如下：

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)

比較表達式和搜索字，如果匹配，返回結果；如果不匹配，返回default值；如果未定義default值，則返回空值。

二、示例：

創建表：

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

插入數據：

insert into student_score (name,subject,score) values ('張三','中文',90);
insert into student_score (name,subject,score) values ('李四','數學',80);
insert into student_score (name,subject,score) values ('王五','外語',89);
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);

查詢：

select a.*,rowid from student_score a;

---如果學科爲中文，則返回分數，否則返回0
select name,subject,decode(subject,'中文',score,0) from student_score;

---統計中文學科的分數
select name,sum(decode (subject,'中文',score,0)) as 中文 from student_score group by name;

select name,score as 中文 from student_score;

---使用decode函數
select name,
       sum(decode(subject, '中文', score, 0)) as 中文,
       sum(decode(subject, '數學', score, 0)) as 數學,
       sum(decode(subject, '外語', score, 0)) as 外語
  from student_score
 group by name;

---使用case when
select name,
       sum(case
             when subject = '中文' then
              nvl(score, 0)
             else
              0
           end) 中文,
       sum(case
             when subject = '數學' then
              nvl(score, 0)
             else
              0
           end) 數學,
       sum(case
             when subject = '外語' then
              nvl(score, 0)
             else
              0
           end) 外語
  from student_score
 group by name