查询出最大值如果是空值用0代替 （用的DB2）

                        SELECT  MAX(DATE_START) AS effectivestarttime,
                                         RULE AS rule,
                                         CONDITIONS AS condition           
            FROM
                       MGRSCHEMA.APPROVE
            WHERE
                 CONDITIONS = '2001'
                 AND RULE = '2001'
                 GROUP BY 
                 CONDITIONS,
                 RULE

MAX(DATE_START)查询出的最大值为空 用0代替 请各位大神帮帮忙

SELECT
case when MAX(DATE_START)==null then 0 else MAX(DATE_START) AS effectivestarttime end
,RULE AS rule

,CONDITIONS AS condition

FROM
MGRSCHEMA.APPROVE
WHERE
CONDITIONS = '1'
AND RULE = '1'
GROUP BY
CONDITIONS,
RULE