Help with a where statement

I am creating an ASP Application and have the following SQL Statement:

select assetname, serialnumber,parentserialnumber,(case when parentserialnumber = '' then 'False' else 'True' end) as assigned
from assets
where parentserialnumber like @asset

So what I have is a drop down box that allows the user to select:

Both
Assigned
Un-Assigned

The problem I am having is how to structure my SQL so that I can take advantage of the 3 possible states. Both would simply be a '%' while Un-Assigned would be ''

But how would I go about getting the Assigned Values, as I am not sure  how I could tweak my SQL statement to allow this.

Cheers

<asp:DropDownList ID="ddlAssigned" runat="server" AppendDataBoundItems="True" 
                            AutoPostBack="true" Width="150px">
                            <asp:ListItem Value="-1">All</asp:ListItem>
                            <asp:ListItem Value="1">Assigned</asp:ListItem>
                            <asp:ListItem Value="2">Un-Assigned</asp:ListItem>
                        </asp:DropDownList>
 
 
//in the code behind   
 
 
 
 Protected Sub SqlDataSource1_Selecting(ByVal sender As Object, ByVal e As SqlDataSourceSelectingEventArgs)
 
       If ddlAssigned.SelectedValue.Equals("1") Then
            e.Command.Parameters("@assigned").Value = ""
        ElseIf ddlAssigned.SelectedValue.Equals("2") Then
            e.Command.Parameters("@assigned").Value = ""
        End If
    End Sub

Select allOpen in new window