DECODE函數和CASE WHEN 比較

http://blog.csdn.net/zhangbingtao2011/article/details/51384393

一，DECODE函數

 

其基本語法為：

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

表示如果value等於if1時，DECODE函數的結果返回then1,...,如果不等於任何一個if值，則返回else。亦即：decode(條件,值1,翻譯值1,值2,翻譯值2,...值n,翻譯值n,缺省值)

延伸用法：

1. 與sign函數聯用比較大小：

select decode(sign(arg1-arg2),-1, arg1, arg2) from dual; --get arg1與arg2的較小值
select decode(sign(3-5),1 ,3, 5) from dual

注：sign()函數根據某個值是0、正數還是負數，分別返回0、1、-1

2. 表、視圖結構轉化：

基本思路：使用substrb函數實現對字段的判斷，然后用decode函數對數據進行重新計算，並生成新的數據和構成新的表(table or view)。

二，CASE WHEN

其語法如下：

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

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 表結構：

 

rma_center | name

---------------------------

123 |qw

23 |ASde

45 |sssdf

55 |e3fbg

55555 |adfv

22221 |sdfsfe

4 |sdfeg

579 |lojgdex

 

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 與CASE WHEN 的比較

　１，DECODE Oracle 特有;
　２，CASE WHEN Oracle , SQL Server, MySQL 都可用;
　３，DECODE 只能用做相等判斷,但是可以配合sign函數進行大於，小於，等於的判斷,CASE 　可用於=,>=,<,<=,<>,is null,is not null 等的判斷;

４，DECODE 使用其來比較簡潔，CASE 雖然復雜但更為靈活;