SUM某个字段结果，如果为Null时则用0插入 如何显示字符转化为number有误

SUM某个字段结果，如果为Null时则用0插入 怎么显示字符转化为number有误？
EG：  
UPDATE   BatchTicketDetail   SET   BatchTicketDetail_Waste   =   (Select   Case   Sum(BatchDetailBad_Qty)   When   'null '   Then   '0 '   else   Sum(BatchDetailBad_Qty)   end   AS   wQty   From   BatchDetailBad

Thanks!!!

------解决方案--------------------
--将 'null ' 改为null
UPDATE BatchTicketDetail SET BatchTicketDetail_Waste = (Select Case Sum(BatchDetailBad_Qty) When null Then '0 ' else Sum(BatchDetailBad_Qty) end AS wQty From BatchDetailBad

------解决方案--------------------
自己解决．．呵呵！就是有点疑问，会用case　when　居然不会用ISNULL?没想到吧！