asp.net - SqlDataSource 使用或不使用参数填充下拉列表

Question

当用户选择日期时，我使用 SqlDataSource 控件列出搜索结果，但如果日期为空，则它将列出所有记录。sql命令怎么做？

SELECT Ref_No as name, Job_Order_ID as value 
FROM Job_Order 
WHERE (Status <> 'JO_Completed') AND (Delivery_Date = @jaDate) 
OR (Status <> 'JO_Completed') ORDER BY Ref_No 

生成下拉列表的代码

 <asp:SqlDataSource ID="NewJobOrderDS" runat="server" SelectCommand="SELECT Ref_No as name, Job_Order_ID as value FROM Job_Order WHERE (Status <> 'JO_Completed') AND (Delivery_Date = @jaDate) ORDER BY Ref_No" OnSelecting="NewJobOrderDS_Selecting">
                                  <SelectParameters>
                                        <asp:ControlParameter ControlID="txtJADate2" Name="jaDate" PropertyName="Text" Type="DateTime"/>   
                                   </SelectParameters>
                                </asp:SqlDataSource>