spark dataframe如何排序？

如何根据dataframe中的某列数据进行排序？如：

 val spark = SparkSession
      .builder()
      .appName("Test")
      .master("local[4]")
      .getOrCreate()

    val sqlContext = spark.sqlContext

    var df:DataFrame = sqlContext.createDataFrame(List(
      ("a", "Alice", 34),
      ("b", "Bob", 36),
      ("c", "Charlie", 30)
    )).toDF("id", "name", "age")


    spark.stop()