DB2 DECODE 函数的用法

在查询数据，需要进行条件判断时，一般我们使用CASE...WHEN实现，当判断条件为相等时，除了使用CASE...WHEN实现，还可以使用DECODE函数。若要使用like、>、<等其他判断条件时，就只能使用CASE...WHEN实现了。下面就解释下DECODE()函数的用法。

DECODE()使用方法： 

decode(条件,值1,翻译值1,值2,翻译值2,...值n,翻译值n,缺省值)

DECODE()含义说明：
IF 条件=值1 THEN
　　　　RETURN(翻译值1)
ELSIF 条件=值2 THEN
　　　　RETURN(翻译值2)
　　　　......
ELSIF 条件=值n THEN
　　　　RETURN(翻译值n)

ELSE
　　　　RETURN(缺省值)
END IF

附上DBCODE（）官方说明

：

DECODE scalar function

                           .--------------------------------------.                              
                           V                                      |                              
>>-DECODE--(--expression1----,--expression2--,--result-expression-+--+--------------------+--)-><
                                                                     '-,--else-expression-'      

The schema is SYSIBM.

The DECODE function compares each expression2 to expression1. If expression1 is equal to expression2, or both expression1 andexpression2 are null, the value of the following result-expresssion is returned. If no expression2 matches expression1, the value ofelse-expression is returned; otherwise a null value is returned.

The  DECODE function is similar to the CASE expression except for the handling of null values:

• A null value of expression1 will match a corresponding null value of expression2.
• If the NULL keyword is used as an argument in the DECODE function, it must be cast to an appropriate data type.

The rules for determining the result type of a  DECODE expression are based on the corresponding CASE expression.

Examples:

The  DECODE expression:

   DECODE (c1, 7, 'a', 6, 'b', 'c')

achieves the same result as the following CASE expression:

   CASE c1
     WHEN 7 THEN 'a'
     WHEN 6 THEN 'b'
     ELSE 'c'
   END

Similarly, the  DECODE expression:

   DECODE (c1, var1, 'a', var2, 'b')

where the values of c1, var1, and var2 could be null values, achieves the same result as the following CASE expression:

   CASE
     WHEN c1 = var1 OR (c1 IS NULL AND var1 IS NULL) THEN 'a'
     WHEN c1 = var2 OR (c1 IS NULL AND var2 IS NULL) THEN 'b'
     ELSE NULL
   END

Consider also the following query:

   SELECT ID, DECODE(STATUS, 'A', 'Accepted',
                             'D', 'Denied',
                             CAST(NULL AS VARCHAR(1)), 'Unknown',
                             'Other')
   FROM CONTRACTS

Here is the same statement using a CASE expression:

   SELECT ID,
     CASE
       WHEN STATUS = 'A' THEN 'Accepted'
       WHEN STATUS = 'D' THEN 'Denied'
       WHEN STATUS IS NULL THEN 'Unknown'
       ELSE 'Other'
     END
   FROM CONTRACTS