dt = MycSQL.Query("SELECT person_cod FROM Min1");

...
...
        private void button1_Click(object sender, EventArgs e)
        {      

            foreach (DataRow row in dt.Rows)
            {
                
                string idFromMin1 = "";
                idFromMin1 = row[0].ToString();
                string idFromAll = "";
               
                idFromAll = MycSQL.GetField("SELECT id_code  FROM TableAll WHERE id_code = '" + idFromMin1 + "' ");
         
                
            
                if (idFromMin1 == idFromAll)
               
                {
                   
                    MycSQL.SetCommand("UPDATE TableAll SET minn = 'min_1' WHERE id_code = '" + idFromMin1 + "'");

                }

            }
        }