how to update rank in c#

hi,
I want rank the flats based on the (Hospitals, Restaurants, Transports, Markets)iscore
I use this coding line .if I rank single flat this code is work but if I rank bulk data(select flatid in drop down) it update the rank in all flatshow can solve this problem

I used this stored procedure

Create proc [dbo].[sp_FlatRank](@flatID varchar(50))
as
update Flats set FlatRank=(select
ISNULL((select max(IScore) from Flats f inner JOIN Hospitals h on f.FlatID=h.FlatID),0)+
ISNULL((select max(IScore)from Flats f inner JOIN Restaurants r on f.FlatID=r.FlatID),0)+
ISNULL((select max(IScore) from Flats f inner JOIN Transports t on f.FlatID=t.FlatID),0)+
ISNULL((select max(IScore) from Flats f inner JOIN Markets m on f.FlatID=m.FlatID),0) )
where flatId=@flatId

this my ranking code

if (con_db.Connect_Database() == true)
        {
            con_db.con.Open();
            SqlCommand cmd = new SqlCommand("select FlatRank from flats where flatid='"+ddlrank.SelectedItem.Text+"'",con_db.con);
            SqlDataReader sdr = cmd.ExecuteReader();
            if (sdr.Read() == true)
            {
                string rank = sdr[0].ToString();
                if (rank == "0" || rank =="")
                {
                    
                    sdr.Dispose();
                    SqlCommand cmd1 = new SqlCommand("sp_FlatRank", con_db.con);
                    cmd1.CommandType = CommandType.StoredProcedure;
                    cmd1.Parameters.AddWithValue("@FlatID", ddlrank.SelectedItem.Text);
                    cmd1.ExecuteNonQuery();
                    SqlDataAdapter da = new SqlDataAdapter("select FlatID,Flatname,FlatLocation,FlatSize,FlatPrice,FlatStatus,DistanceRef,Distance, ROUND(FlatRank , 0)as FlatRank from  flats where FlatID='" + ddlrank.SelectedItem.Text + "'", con_db.con);
                    DataSet ds = new DataSet();
                    da.Fill(ds, "Flats");
                    GridView1.DataSource = ds.Tables["Flats"];
                    GridView1.DataBind();
                    
                    
                }
                else
                {
                     sdr.Dispose();
                    SqlDataAdapter da = new SqlDataAdapter("select FlatID,Flatname,FlatLocation,FlatSize,FlatPrice,FlatStatus,DistanceRef,Distance, ROUND(FlatRank , 0)as FlatRank from  flats where FlatID='" + ddlrank.SelectedItem.Text + "'", con_db.con);
                    DataSet ds = new DataSet();
                    da.Fill(ds, "Flats");
                    GridView1.DataSource = ds.Tables["Flats"];
                    GridView1.DataBind();
                }
            }
            con_db.con.Close();   
                       
        }
        else
        {
            Response.Write("The Needed database is not connected");
        }

regards
sekar.c

Regards
Sekar.c