oracle 学习笔记 - DECODE函数和CASE WHEN 比较

爱被打了一巴掌 2022-06-10 10:37 351阅读 0赞

## 一，DECODE函数 ##

**其基本语法为：**

Sql代码   ![收藏代码][Image 1]![Image 1][] [![在CODE上查看代码片][CODE]][CODE_CODE] [![派生到我的代码片][ico_fork.svg]][ico_fork.svg 1]

1.  DECODE(value, if1, then1, if2, then2, if3, then3,…ifn, thenn, else)

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**\[sql\]** view plain copy

print ?

1.  DECODE(value, if1, then1, if2, then2, if3, then3,…ifn, thenn, else)

DECODE(value, if1, then1, if2, then2, if3, then3,...ifn, thenn, else)

表示如果value等于if1时，DECODE函数的结果返回then1,…,如果不等于任何一个if值，则返回else。亦即：decode(条件,值1,翻译值1,值2,翻译值2,…值n,翻译值n,缺省值)

**延伸用法：**

***1. 与sign函数联用比较大小：***

Sql代码   ![收藏代码][Image 1]![Image 1][] [![在CODE上查看代码片][CODE]][CODE_CODE] [![派生到我的代码片][ico_fork.svg]][ico_fork.svg 1]

1.  select decode(sign(arg1-arg2),-1, arg1, arg2) from dual; –get arg1与arg2的较小值

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**\[sql\]** view plain copy

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1.  select decode(sign(arg1-arg2),-1, arg1, arg2) from dual; –get arg1与arg2的较小值

select decode(sign(arg1-arg2),-1, arg1, arg2) from dual; --get arg1与arg2的较小值

Sql代码   ![收藏代码][Image 1]![Image 1][] [![在CODE上查看代码片][CODE]][CODE_CODE] [![派生到我的代码片][ico_fork.svg]][ico_fork.svg 1]

1.  select decode(sign(3-5),1 ,3, 5) from dual

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**\[sql\]** view plain copy

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1.  select decode(sign(3-5),1 ,3, 5) from dual

select decode(sign(3-5),1 ,3, 5) from dual

***注：***sign()函数根据某个值是0、正数还是负数，分别返回0、1、-1

***2. 表、视图结构转化：***

**基本思路：**使用substrb函数实现对字段的判断，然后用decode函数对数据进行重新计算，并生成新的数据和构成新的表(table or view)。

## 二，CASE WHEN ##

其语法如下：

Sql代码   ![收藏代码][Image 1]![Image 1][] [![在CODE上查看代码片][CODE]][CODE_CODE] [![派生到我的代码片][ico_fork.svg]][ico_fork.svg 1]

1.  SELECT <myColumnSpec> =   
2.  　　CASE WHEN <A> THEN <somethingA>   
3.  　　　　　WHEN <B> THEN <somethingB>   
4.  　　　　　ELSE <somethingE> END  
5.

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**\[sql\]** view plain copy

print ?

1.  SELECT <myColumnSpec> =   
2.  　　CASE WHEN <A> THEN <somethingA>   
3.  　　　　　WHEN <B> THEN <somethingB>   
4.  　　　　　ELSE <somethingE> END  
5.

SELECT <myColumnSpec> = 
    　　CASE WHEN <A> THEN <somethingA> 
    　　　　　WHEN <B> THEN <somethingB> 
    　　　　　ELSE <somethingE> END

除了可以在select 中使用CASE 外，where 子句,group by 子句,order by 子句都可以使用

Sql代码   ![收藏代码][Image 1]![Image 1][] [![在CODE上查看代码片][CODE]][CODE_CODE] [![派生到我的代码片][ico_fork.svg]][ico_fork.svg 1]

1.  SELECT     CASE           
2.              WHEN price IS NULL THEN ‘Unpriced’           
3.              WHEN price < 10 THEN ‘Bargain’           
4.              WHEN price BETWEEN 10 and 20 THEN ‘Average’           
5.              ELSE ‘Gift to impress relatives’       
6.             END AS “Range”,       
7.             Title   
8.  FROM titles   
9.  where   
10.     CASE           
11.                 WHEN price IS NULL THEN ‘Unpriced’           
12.                 WHEN price < 10 THEN ‘Bargain’           
13.                 WHEN price BETWEEN 10 and 20 THEN ‘Average’           
14.                 ELSE ‘Gift to impress relatives’     END in(‘Average’,‘Bargain’)  
15. GROUP BY     CASE           
16.                 WHEN price IS NULL THEN ‘Unpriced’           
17.                 WHEN price < 10 THEN ‘Bargain’           
18.                 WHEN price BETWEEN 10 and 20 THEN ‘Average’           
19.                 ELSE ‘Gift to impress relatives’     END,       
20.                 Title   
21. ORDER BY     CASE           
22.                 WHEN price IS NULL THEN ‘Unpriced’           
23.                 WHEN price < 10 THEN ‘Bargain’           
24.                 WHEN price BETWEEN 10 and 20 THEN ‘Average’           
25.                 ELSE ‘Gift to impress relatives’       
26.                 END,Title

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**\[sql\]** view plain copy

print ?

SELECT     CASE         
                WHEN price IS NULL THEN 'Unpriced'         
                WHEN price < 10 THEN 'Bargain'         
                WHEN price BETWEEN 10 and 20 THEN 'Average'         
                ELSE 'Gift to impress relatives'     
               END AS "Range",     
               Title 
    FROM titles 
    where 
        CASE         
                    WHEN price IS NULL THEN 'Unpriced'         
                    WHEN price < 10 THEN 'Bargain'         
                    WHEN price BETWEEN 10 and 20 THEN 'Average'         
                    ELSE 'Gift to impress relatives'     END in('Average','Bargain')
    GROUP BY     CASE         
                    WHEN price IS NULL THEN 'Unpriced'         
                    WHEN price < 10 THEN 'Bargain'         
                    WHEN price BETWEEN 10 and 20 THEN 'Average'         
                    ELSE 'Gift to impress relatives'     END,     
                    Title 
    ORDER BY     CASE         
                    WHEN price IS NULL THEN 'Unpriced'         
                    WHEN price < 10 THEN 'Bargain'         
                    WHEN price BETWEEN 10 and 20 THEN 'Average'         
                    ELSE 'Gift to impress relatives'     
                    END,Title

**rm\_site\_master 表结构：**

**rma\_center | name**

**—————————**

123              **|**qw

23                **|**ASde

45                **|**sssdf

55                **|**e3fbg

55555          **|**adfv

22221          **|**sdfsfe

4                  **|**sdfeg

579              **|**lojgdex

Sql代码   ![收藏代码][Image 1]![Image 1][] [![在CODE上查看代码片][CODE]][CODE_CODE] [![派生到我的代码片][ico_fork.svg]][ico_fork.svg 1]

1.  select name,  
2.         CASE           
3.          WHEN rma\_center IS NULL THEN ‘Null’           
4.          WHEN rma\_center > 1000 THEN ‘>1000’           
5.          WHEN rma\_center BETWEEN 30 and 100 THEN ‘30~100’           
6.          ELSE ‘Gift to impress relatives’     END  AS “RMA CENTER Type”                      
7.  from rm\_site\_master   
8.  where  CASE           
9.          WHEN rma\_center IS NULL THEN ‘Null’           
10.         WHEN rma\_center > 1000 THEN ‘>1000’           
11.         WHEN rma\_center BETWEEN 30 and 100 THEN ‘30~100’           
12.         ELSE ‘Gift to impress relatives’     END in(‘30~100’,‘>1000’)  
13. group by CASE           
14.         WHEN rma\_center IS NULL THEN ‘Null’           
15.         WHEN rma\_center > 1000 THEN ‘>1000’           
16.         WHEN rma\_center BETWEEN 30 and 100 THEN ‘30~100’           
17.         ELSE ‘Gift to impress relatives’     END,  
18.         name  
19. order by CASE           
20.         WHEN rma\_center IS NULL THEN ‘Null’           
21.         WHEN rma\_center > 1000 THEN ‘>1000’           
22.         WHEN rma\_center BETWEEN 30 and 100 THEN ‘30~100’           
23.         ELSE ‘Gift to impress relatives’     END,  
24.         name

![save_snippets.png][]

**\[sql\]** view plain copy

print ?

select name,
           CASE         
            WHEN rma_center IS NULL THEN 'Null'         
            WHEN rma_center > 1000 THEN '>1000'         
            WHEN rma_center BETWEEN 30 and 100 THEN '30~100'         
            ELSE 'Gift to impress relatives'     END  AS "RMA CENTER Type"                    
    from rm_site_master 
    where  CASE         
            WHEN rma_center IS NULL THEN 'Null'         
            WHEN rma_center > 1000 THEN '>1000'         
            WHEN rma_center BETWEEN 30 and 100 THEN '30~100'         
            ELSE 'Gift to impress relatives'     END in('30~100','>1000')
    group by CASE         
            WHEN rma_center IS NULL THEN 'Null'         
            WHEN rma_center > 1000 THEN '>1000'         
            WHEN rma_center BETWEEN 30 and 100 THEN '30~100'         
            ELSE 'Gift to impress relatives'     END,
            name
    order by CASE         
            WHEN rma_center IS NULL THEN 'Null'         
            WHEN rma_center > 1000 THEN '>1000'         
            WHEN rma_center BETWEEN 30 and 100 THEN '30~100'         
            ELSE 'Gift to impress relatives'     END,
            name

## 三，DECODE 与CASE WHEN 的比较 ##

１，DECODE  [Oracle][] 特有;  
　２，CASE WHEN  [oracle][Oracle] ,  SQL Server,  [MySQL][] 都可用;  
　３，DECODE 只能用做相等判断,但是可以配合sign函数进行大于，小于，等于的判断,CASE 　可用于=,>=,<,<=,<>,is null,is not null，between … and … 等的判断;

４，CASE when 可以做等值判断，也可以带入表达式判断，更为灵活;

SQL> select case instr(hiredate,'82')
      2  when 0 then 0
      3  else 1
      4  end as "year"
      5  from emp;
    
          year
    ----------
             0
             0
             0
             0
             0
             0
             0
             0
             0
             0
             0
             0
             0
             1

SQL> select count(*) total,
      2  sum(case
      3  when instr(hiredate,'80')=0 then 0
      4  else 1
      5  end
      6  ) as "1980",
      7  sum(case
      8  when instr(hiredate,'81')=0 then 0
      9  else 1
     10  end
     11  ) as "1981",
     12  sum(case
     13  when instr(hiredate,'82')=0 then 0
     14  else 1
     15  end
     16  ) as "1982",
     17  sum(case
     18  when instr(hiredate,'87')=0 then 0
     19  else 1
     20  end
     21  ) as "1987"
     22  from emp;
    
         TOTAL       1980       1981       1982       1987
    ---------- ---------- ---------- ---------- ----------
            14          1         10          1          2

5，DECODE 使用其来比较简洁，CASE 虽然复杂但更为灵活;

转自：[http://blog.csdn.net/nosodeep/article/details/21232023][http_blog.csdn.net_nosodeep_article_details_21232023]

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