<asp:Content ID="BodyContent" runat="server" ContentPlaceHolderID="MainContent">
          <asp:DropDownList ID="DropDownList1" runat="server" 
            DataSourceID="SqlDataSource1" DataTextField="qwe" DataValueField="qwe" 
              AutoPostBack="True">
          <asp:listitem selected="True">Sales Representative</asp:listitem>
          <asp:listitem>Sales Manager</asp:listitem>
          <asp:listitem>Vice President, Sales</asp:listitem>
          </asp:DropDownList>
   
          

 <asp:SqlDataSource ID="SqlDataSource1" runat="server" 
            ConnectionString="<%$ ConnectionStrings:Database1ConnectionString %>" 
            SelectCommand="SELECT [id], [qwe], [asd] FROM [Table2]"
            FilterExpression="qwe='{0}'">              
                           <FilterParameters>
                     <asp:ControlParameter Name="qwe" ControlId="DropDownList1" PropertyName="SelectedValue"/>     
                            
                </FilterParameters>
</asp:SqlDataSource>
    <asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False" 
              DataSourceID="SqlDataSource1" DataKeyNames="id">
        <Columns>     
        <asp:BoundField DataField="id" HeaderText="id" 
                SortExpression="id" ReadOnly="True" /> 
                <asp:BoundField DataField="qwe" HeaderText="qwe" 
                SortExpression="qwe" />      
           <asp:BoundField DataField="asd" HeaderText="asd" 
                SortExpression="asd" /> 
        </Columns>
    </asp:GridView>
</asp:Content>