使用CASE作查询条件后，怎么速度变得很慢，应怎样解决？

[fs_hzj]

Select * from vwOutHouseDsp
where iDocStatus<=100 and dDocDate>=@tdBDate and dDocDate<=@tdEDate
and (cDocType='销售发货' or cDocType='销售退货')
and WhSysCode=case when @tcWhSysCode='' then WhSysCode else @tcWhSysCode End
and ISysCode=case when @tcISysCode='' then ISysCode else @tcISysCode End
and IBSysCode=case when @tcIBSysCode='' then IBSysCode else @tcIBSysCode

以上这句在WHERE上使用CASE后查速度就变得很慢了，如果把
and WhSysCode=case when @tcWhSysCode='' then WhSysCode else @tcWhSysCode End
and ISysCode=case when @tcISysCode='' then ISysCode else @tcISysCode End
and IBSysCode=case when @tcIBSysCode='' then IBSysCode else @tcIBSysCode
这几句去掉，速度就正常了，怎么会这样呢？
另外，如果不用CASE，这种情况的条件查询怎样解决比较好呢？

[stou]

语句会出错的。

[wwwwb]

用IIF试试
WhSysCode=iif(@tcWhSysCode='',WhSysCode,@tcWhSysCode)
and ISysCode=iif(@tcISysCode='',ISysCode,@tcISysCode)
and IBSysCode=iif(@tcIBSysCode='',IBSysCode,@tcIBSysCode )

[hrb2008]

把条件
@tcWhSysCode,@tcISysCode,@tcIBSysCode 默认设成null
create procedure protest
@tcWhSysCode int =null,
@tcISysCode int =null,
@tcIBSysCode int =null
as
Select * from vwOutHouseDsp
where iDocStatus<=100 and dDocDate>=@tdBDate and dDocDate<=@tdEDate
and (cDocType='销售发货' or cDocType='销售退货')
and WhSysCode=isnull(@tcWhSysCode,WhSysCode)
and ISysCode=isnull(@tcISysCode,ISysCode)
and IBSysCode=isnull(@tcIBSysCode,IBSysCode)

[xiaoxiao8310]

好象写的不正确啊，case那些应该放在select后面吧