GridView and SqlDataSoruce SORTING issue - asp.net

Question

I have got very basic Product data table, I set up my grid view data source to sqlDataSoruce with paging and sorting options true.
But I also have a DropDownList and if a user chooses a certain product group ( Vegetables, Fruits, Frozen etc), grid view only shows that product group items.
String sqlSearchByProduct = " SELECT ID, pID,pGroupID, pName, pPrice, Status, Stock FROM productsTable WHERE (pGroupID= ProductGroupIDDropDownList.SelectedItem.Value)";
sqlDataSource.SelectCommand = sqlSearchByProduct ;
The grid shows all products from groupID selected from dropdownlist but the issue is when I want to sort it by name or price, grid view shows all products again.
I will be very grateful if you know how to fix this issue.
Thank you.

Answer 1

You need to keep SelectedValue of the dropdown and the easiest way to do so is to bind all data declaratively. Something like this:
<asp:DropDownList ID="ddCategory" runat="server" DataTextField="categoryname"
DataValueField="categoryid"
AppendDataBoundItems="true" DataSourceID="sqlCategory" AutoPostBack="true">
<asp:ListItem Value="0" Text="- Select -"></asp:ListItem>
</asp:DropDownList>
<asp:SqlDataSource ID="sqlCategory" runat="server" ConnectionString="<%$ ConnectionStrings:NorthwindCnn %>"
SelectCommand="select categoryid,categoryname from dbo.categories">
</asp:SqlDataSource>
<asp:GridView ID="gvProducts" runat="server" AutoGenerateColumns="False" AllowPaging="True" PageSize="5" DataKeyNames="ProductID" DataSourceID="sqlNWind">
<Columns>
<asp:BoundField DataField="ProductID" HeaderText="ProductID" InsertVisible="False" ReadOnly="True" SortExpression="ProductID" />
<asp:BoundField DataField="ProductName" HeaderText="ProductName" SortExpression="ProductName" />
</Columns>
</asp:GridView>
<asp:SqlDataSource ID="sqlNWind" runat="server" ConnectionString="<%$ ConnectionStrings:NorthwindCnn %>"
SelectCommand="SELECT Products.ProductID, Products.ProductName FROM Products where (#categoryid=0 or CategoryID=#categoryId)">
<SelectParameters>
<asp:ControlParameter Name="categoryid" ControlID="ddCategory" Type="Int32" PropertyName="SelectedValue" />
</SelectParameters>
</asp:SqlDataSource>
No code behind required.
Side note: never build sql query string like this
" ... WHERE (pGroupID= ProductGroupIDDropDownList.SelectedItem.Value)"
Such code is prone to SQL Injection attack. Always use parameters.

Answer 2

Have you rebound your data to the gridview, e.g. below
GridView1.DataBind();