DB2 DECODE函數的用法 在查詢數據，需要進行條件判斷時，一般我們使用CASE...WHEN實現，當判斷條件為相等時，除了使用CASE...WHEN實現，還可以使用DECODE函數。若要使用like、>、<等其他判斷條件時，就只能使用CASE...WHEN實現了。下面就解釋下DECODE()函數的用法。 DECODE()使用方法： decode(條件,值1,翻譯值1,值2,翻譯值2,...值n,翻譯值n,缺省值) www.2cto.com DECODE()含義說明： IF 條件=值1 THEN RETURN(翻譯值1) ELSIF 條件=值2 THEN RETURN(翻譯值2) ...... ELSIF 條件=值n THEN RETURN(翻譯值n) ELSE RETURN(缺省值) END IF www.2cto.com 附上DBCODE（）官方說明 ： 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
