code: function getMinWlByRoundIndex(conn, roundIndex, callback){
      conn.query(
  		"SELECT min_wl FROM round where round_index=?", 
          [roundIndex],
  		function(rows){
  			if (rows.length !== 1)
                  throw Error("Can not find the right round index");
              callback(rows[0].min_wl);
  		}
  	);
  }
docs: the MinWl maybe null