SELECT query with querystring parameters using NULL value - asp.net

Question

Hello to all respected members,
Presently I using following SELECT command in asp.net for populating a Gridview based on Query string values:
SelectCommand="SELECT ID, Date, Train, I_R, Dir_Ind, Detn, Rly, DiV, Loco, Shed, locoClass, loco_type FROM PunctualityMain WHERE Date >= #strdt AND Date <= #enddt AND ([Rly] = #rly OR #rly IS NULL) AND ([shed] = #shed OR #shed IS NULL) Order by Date ASC">
<SelectParameters>
<asp:QueryStringParameter Name="strdt" QueryStringField="strdt" Type="DateTime" />
<asp:QueryStringParameter Name="enddt" QueryStringField="enddt" Type="DateTime" />
<asp:QueryStringParameter Name="rly" QueryStringField="rly" Type="string" />
<asp:QueryStringParameter Name="shed" QueryStringField="shed" Type="string" />
</SelectParameters>
It is working fine also with two values null as :
http://elocos.railnet.gov.in/Punctuality/Punctualitydetail.aspx?strdt=5/1/2013&enddt=5/14/2013&rly=&shed=
Now I want to use same for Gridview with Delete, Insert & Update Commands which is as under:
DeleteCommand="DELETE FROM [PunctualityMain] WHERE [ID] = #ID"
InsertCommand="INSERT INTO [PunctualityMain] ([Date], [Train], [Dir_Ind], [Detn], [Rly], [DiV], [Loco], [Shed], [locoClass], [loco_type]) VALUES (#Date, #Train, #Dir_Ind, #Detn, #Rly, #DiV, #Loco, #Shed, #locoClass, #loco_type)"
SelectCommand="SELECT * FROM [PunctualityMain] WHERE (([Date] >= #strdt) AND ([Date] <= #enddt) AND ([Shed] = #shed OR #shed IS NULL) AND ([Rly] = #rly OR #rly IS NULL)) ORDER BY [Date]"
UpdateCommand="UPDATE [PunctualityMain] SET [Date] = #Date, [Train] = #Train, [Dir_Ind] = #Dir_Ind, [Detn] = #Detn, [Rly] = #Rly, [DiV] = #DiV, [Loco] = #Loco, [Shed] = #Shed, [locoClass] = #locoClass, [loco_type] = #loco_type WHERE [ID] = #ID">
<DeleteParameters>
<asp:Parameter Name="ID" Type="Int32" />
</DeleteParameters>
<InsertParameters>
<asp:Parameter Name="Date" Type="DateTime" />
<asp:Parameter Name="Train" Type="Int32" />
<asp:Parameter Name="Dir_Ind" Type="String" />
<asp:Parameter Name="Detn" Type="Int32" />
<asp:Parameter Name="Rly" Type="String" />
<asp:Parameter Name="DiV" Type="String" />
<asp:Parameter Name="Loco" Type="String" />
<asp:Parameter Name="Shed" Type="String" />
<asp:Parameter Name="locoClass" Type="String" />
<asp:Parameter Name="loco_type" Type="String" />
</InsertParameters>
<SelectParameters>
<asp:QueryStringParameter Name="strdt" QueryStringField="strdt" Type="DateTime" />
<asp:QueryStringParameter Name="enddt" QueryStringField="enddt" Type="DateTime" />
<asp:QueryStringParameter Name="rly" QueryStringField="rly" Type="string" />
<asp:QueryStringParameter Name="shed" QueryStringField="shed" Type="string" />
</SelectParameters>
<UpdateParameters>
<asp:Parameter Name="Date" Type="DateTime" />
<asp:Parameter Name="Train" Type="Int32" />
<asp:Parameter Name="Dir_Ind" Type="String" />
<asp:Parameter Name="Detn" Type="Int32" />
<asp:Parameter Name="Rly" Type="String" />
<asp:Parameter Name="DiV" Type="String" />
<asp:Parameter Name="Loco" Type="String" />
<asp:Parameter Name="Shed" Type="String" />
<asp:Parameter Name="locoClass" Type="String" />
<asp:Parameter Name="loco_type" Type="String" />
<asp:Parameter Name="ID" Type="Int32" />
</UpdateParameters>
Why the above SELECT command not working with NULL values for rly & shed? However, with all the four query string values it is working like this
http://elocos.railnet.gov.in/Punctuality/Punctualitydetail.aspx?strdt=5/1/2013&enddt=5/14/2013&rly=CR&shed=KYN
Please suggest any solution for the problem so that it can work with this string also (NULL string values)
http://elocos.railnet.gov.in/Punctuality/Punctualitydetail.aspx?strdt=5/1/2013&enddt=5/14/2013&rly=&shed=

Answer 1

Because you get Empty value from empty querystring value =/= null
In order to get this working you should set null value manually.
I can suggest you at least 2 ways:
1) In codebehind:
string rly = Request.QueryString["rly"];
if (rly == "")
rly = null;
And then use it.
2) In SQL command:
Rly=NullIf('#rly','')
But I'm not sure if 2nd one will work like this, if it won't then you can try to workaround it like:
codebehind string empt = "";
SQL command Rly=NullIf('#rly','+empt+')
Or you can go some other way, the point is to verify it "rly" query is "", if it is then set it to null, otherwise leave it alone.