Gridview row id

I have Gridview full with data and I need when some of rows are checked i can be able to press the button and change the data accordingly, I can do it when all rows are checked but i can't find a way to connect my queries with id of one or many rows.

protected void UsedForButton_Click(object sender, EventArgs e) { foreach (GridViewRow row in GridView1.Rows) { var chk = row.FindControl("InStockCbx") as CheckBox;

            if (chk.Checked)
            {
                //using (WorkDBEntities db = new WorkDBEntities())
                //{

                //    WorkTable table = new WorkTable();
                //    table.InStock = 0;

                //    db.WorkTables.Add(table);
                //    db.SaveChanges();    
                //}

                string query = "UPDATE WorkTable SET InStock = 0 WHERE Id = " ???;

                SqlCommand comm = new SqlCommand(query, conn);
                conn.Open();
                comm.ExecuteNonQuery();
                conn.Close();
            }