How can I show multiple database columns in gridview databound column?

Solution:

GridView BoundField can only bind to a single field. If you want to do this, you can do so by one of the following ways:
> use calculated column in your database query i.e. concatenate multiple columns to a single column. For example: Select housenumber+', '+street as Address from client
 
> use GridView TemplateField. You can bind individual template in a single TempalteField. Following code snippet shows the trick:

<asp:GridView ID="GridView1" runat="server">
    <Columns>
        <asp:TemplateField HeaderText="Address">
            <ItemTemplate>
                <%#Eval("housenumber")%>, <%#Eval("street")%>
            </ItemTemplate>
        </asp:TemplateField>
    </Columns>
</asp:GridView>