Its called master detail concept: Try this code
<%--Master GridView--%>
<asp:GridView ID="GridView1" runat="server" DataSourceID="SqlDataSource3" DataKeyNames="empid"
OnSelectedIndexChanged="GridView1_SelectedIndexChanged">
<Columns>
<asp:CommandField ShowSelectButton="True" />
<asp:BoundField DataField="empid" HeaderText="empid" SortExpression="empid" />
<asp:BoundField DataField="ename" HeaderText="ename" SortExpression="ename" />
<asp:BoundField DataField="bdate" HeaderText="bdate" SortExpression="bdate" />
</Columns>
</asp:GridView>
<asp:SqlDataSource ID="SqlDataSource3" runat="server"
ConnectionString="<%$ ConnectionStrings:ConnectionString %>"
SelectCommand="SELECT * FROM [emp]"></asp:SqlDataSource>
<%-- Details GridView--%>
<asp:GridView ID="GridView2" runat="server" DataSourceID="SqlDataSource4" AutoGenerateColumns="False">
<Columns>
<asp:BoundField DataField="empid" HeaderText="empid" SortExpression="empid" />
<asp:BoundField DataField="ename" HeaderText="ename" SortExpression="ename" />
<asp:BoundField DataField="bdate" HeaderText="bdate" SortExpression="bdate" />
</Columns>
</asp:GridView>
<asp:SqlDataSource ID="SqlDataSource4" runat="server"
ConnectionString="<%$ ConnectionStrings:ConnectionString %>"
SelectCommand="SELECT * FROM [edetail] where empid=@empid">
<SelectParameters>
<asp:ControlParameter ControlID="GridView1" Name="empid" PropertyName="SelectedValue"
Type="Int32" />
</SelectParameters>
</asp:SqlDataSource>