ASP.Net รบกวนเกี่ยวกับการใช้ Gridview Edit หน่อย ครับ CommandField ShowEditButton="true"

<asp:GridView ID="gvStaff" runat="server" Visible="False" 
        AutoGenerateColumns="False" Width="100%" 
        BackColor="LightGoldenrodYellow" BorderColor="Tan" BorderWidth="1px" 
        CellPadding="2" ForeColor="Black" GridLines="None" AllowPaging="True" 
        onpageindexchanging="gvStaff_PageIndexChanging" PageSize="20" 
        onrowcommand="gvGrid_Command">
        <AlternatingRowStyle BackColor="PaleGoldenrod" />
        <FooterStyle BackColor="Tan" />
        <HeaderStyle BackColor="Tan" Font-Bold="True" />
        <PagerStyle BackColor="PaleGoldenrod" ForeColor="DarkSlateBlue" 
            HorizontalAlign="Center" />
        <SelectedRowStyle BackColor="DarkSlateBlue" ForeColor="GhostWhite" />
        <SortedAscendingCellStyle BackColor="#FAFAE7" />
        <SortedAscendingHeaderStyle BackColor="#DAC09E" />
        <SortedDescendingCellStyle BackColor="#E1DB9C" />
        <SortedDescendingHeaderStyle BackColor="#C2A47B" />

        <Columns>

           <asp:BoundField DataField ="CITIZEN_ID" HeaderText="CITIZEN_ID" SortExpression="CITIZEN_ID" ItemStyle-HorizontalAlign="Center" />
           <asp:BoundField DataField ="STF_FNAME" HeaderText="Name" SortExpression="STF_FNAME" ItemStyle-HorizontalAlign="Center" />
           <asp:BoundField DataField ="STF_LNAME" HeaderText="Lastname" SortExpression="STF_LNAME" ItemStyle-HorizontalAlign="Center" />
           <asp:BoundField DataField ="BIRTHDAY" HeaderText="Birthday" SortExpression="BIRTHDAY" ItemStyle-HorizontalAlign="Center" />
           <asp:BoundField DataField ="TELEPHONE" HeaderText="Tel" SortExpression="TELEPHONE" ItemStyle-HorizontalAlign="Center" />
           
           <asp:CommandField ShowEditButton="true" ButtonType="Link" HeaderStyle-Wrap="true" EditText="Edit" />

        </Columns>

    </asp:GridView>

protected void btnType_Click(object sender, EventArgs e)
        {
            if(ddlType.SelectedValue == "1")
            {
                gvStaff.Visible = true;
                gvStudent.Visible = false;

                BindData_gvStaff();
            }
            else if (ddlType.SelectedValue == "2")
            {
                gvStaff.Visible = false;
                gvStudent.Visible = true;

                BindData_gvStudent();
            }
            else
            {
                Page.ClientScript.RegisterStartupScript(this.GetType(), "ErrorAlert", "alert('กรุณาเลือกประเภทข้อมูลที่ต้องการโชว์');", true);
                gvStaff.Visible = false;
                gvStaff.DataSource = null;
                gvStaff.DataBind();

                gvStudent.Visible = false;
                gvStudent.DataSource = null;
                gvStudent.DataBind();

            }
        }

        protected void BindData_gvStaff()
        {
            String strSQL;
            strSQL = "SELECT CITIZEN_ID,STF_FNAME,STF_LNAME,BIRTHDAY,TELEPHONE FROM UOC_STAFF ";

            clsDB.QueryDataTable(strSQL);
            gvStaff.DataSource = clsDB.QueryDataTable(strSQL);
            gvStaff.DataBind();
        }

        protected void BindData_gvStudent()
        {
            String strSQL;
            strSQL = "SELECT STUDENTCODE,CITIZEN_ID,STD_FNAME,STD_LNAME,BIRTHDAY FROM UOC_STUDENT ";

            clsDB.QueryDataTable(strSQL);
            gvStudent.DataSource = clsDB.QueryDataTable(strSQL);
            gvStudent.DataBind();
        }

        protected void gvStaff_PageIndexChanging(object sender, GridViewPageEventArgs e)
        {
            String strSQL;
            strSQL = "SELECT CITIZEN_ID,STF_FNAME,STF_LNAME,BIRTHDAY,TELEPHONE FROM UOC_STAFF";

            clsDB.QueryDataTable(strSQL);
            gvStaff.PageIndex = e.NewPageIndex;
            gvStaff.DataSource = clsDB.QueryDataTable(strSQL);
            gvStaff.DataBind();

            

        }

        protected void gvStudent_PageIndexChanging(object sender, GridViewPageEventArgs e)
        {
            String strSQL;
            strSQL = "SELECT STUDENTCODE,CITIZEN_ID,STD_FNAME,STD_LNAME,BIRTHDAY FROM UOC_STUDENT ";

            clsDB.QueryDataTable(strSQL);
            gvStudent.PageIndex = e.NewPageIndex;
            gvStudent.DataSource = clsDB.QueryDataTable(strSQL);
            gvStudent.DataBind();
        }