orderBy和sort：按指定字段排序，默认为升序 ，例如，按照时间字段进行排序:
df.orderBy("visittime").show(false)
输出：
+---+----+----+---------------------+
|id |user|type|visittime |
+---+----+----+---------------------+
|12 |1 |助手2 |2017-07-07 13:45:08.0|
|10 |2 |助手1 |2017-07-14 13:44:57.0|
|11 |1 |APP1|2017-07-15 13:45:03.0|
|6 |3 |APP1|2017-08-01 13:44:41.0|
|5 |3 |APP1|2017-08-02 13:44:38.0|
|8 |3 |APP2|2017-08-03 13:44:45.0|
|2 |1 |APP1|2017-08-04 13:44:26.0|
|3 |2 |助手1 |2017-08-05 13:44:29.0|
|4 |2 |助手1 |2017-08-07 13:44:32.0|
|1 |1 |助手1 |2017-08-10 13:44:19.0|
|9 |2 |APP2|2017-08-11 13:44:53.0|
|7 |3 |助手2 |2017-08-14 13:44:48.0|
+---+----+----+---------------------+df
