Sql server 2005 编写可怕的SQL搜索查询

我正在处理一个似乎不起作用的搜索查询。 完整的查询是：

set ANSI_NULLS ON
set QUOTED_IDENTIFIER ON
go

ALTER PROCEDURE [dbo].[usp_Item_Search]
    @Item_Num varchar(30) = NULL
    ,@Search_Type int = NULL
    ,@Vendor_Num varchar(10) = NULL
    ,@Search_User_ID int = NULL
    ,@StartDate smalldatetime = NULL
    ,@EndDate smalldatetime = NULL
AS
DECLARE @SQLstr as nvarchar(4000)

Set @SQLstr = 'SELECT RecID, Vendor_Num, Vendor_Name, InvoiceNum, Item_Num, 
(SELECT CONVERT(VARCHAR(11), RecDate, 106) AS [DD MON YYYY]) As RecDate, NeedsUpdate, RecAddUserID FROM [tbl_ItemLog] '

IF (@Item_Num IS NOT NULL)  
    Begin
        If @Search_Type = 0
            BEGIN
                Set @SQLstr = @SQLstr +  'WHERE Item_Num LIKE ''' + @Item_Num + '%'''
            END
        If @Search_Type = 1
            BEGIN
                Set @SQLstr = @SQLstr + 'WHERE Item_Num LIKE ''%' + @Item_Num + '%'''
            END
        If @Search_Type = 2
            BEGIN
                Set @SQLstr = @SQLstr + 'WHERE Item_Num LIKE ''%' + @Item_Num + ''''
            END
    End

IF (@Vendor_Num IS NOT NULL)
    Begin
        Set @SQLstr = @SQLstr + ' AND Vendor_Num = ''' + @Vendor_Num + ''''
    End

IF (Search_User_ID IS NOT NULL)
    Begin
        Set @SQLstr = @SQLstr + ' AND RecAddUserID = ' + convert(nvarchar(20),@Search_User_ID)
    End

Set @SQLstr = @SQLstr + ' AND (RecDate BETWEEN ''' + convert(nvarchar(10),@StartDate,101) + ''' AND ''' + convert(nvarchar(10),@EndDate,101) + ''')'

PRINT (@SQLstr)
--Execute (@SQLstr)

当我向查询传递参数时，如果Vendor_Num值为空，则PRINT（@SQLstr）的输出为

我希望输出是：

SELECT RecID, Vendor_Num, Vendor_Name, InvoiceNum, Item_Num,  
(SELECT CONVERT(VARCHAR(11), RecDate, 106) AS [DD MON YYYY]) As RecDate, 
NeedsUpdate, RecAddUserID FROM [tbl_ItemLog] WHERE Item_Num LIKE '2200%' 
AND RecAddUserID = 2 AND (RecDate BETWEEN '01/01/2001' AND '12/12/2002')

如何构造查询，以便在参数值为空时不显示字段。

尝试更改

IF (@Vendor_Num IS NOT NULL) 

到

如果（@Vendor\u Num不为NULL且LTRIM（@Vendor\u Num）'））

尝试更改

IF (@Vendor_Num IS NOT NULL) 

到

如果（@Vendor\u Num不为NULL且LTRIM（@Vendor\u Num）'））

IF (@Vendor_Num IS NOT NULL and LTRIM(@Vendor_Num) <> '')