1. partition by和group by区别和联系

1）group by是分组函数，partition by是分析函数（然后像sum()等是聚合函数）

2）在执行顺序上partition by应用在以上关键字之后，实际上就是在执行完select之后，在所得结果集之上进行partition，group by 使用常用sql关键字的优先级(from > where > group by > having > order by)

3）partition by相比较于group by，能够在 保留全部数据 的基础上，只对其中某些字段做分组排序，而group by则只保留 参与分组的字段和聚合函数 的结果

2. spark sql 使用group by

    val df = Seq(
      ("ABC", "2019-02-10", "411626"),
      ("ABC", "2019-02-10", "411627"),
      ("BCD", "2020-04-01", "411626"),
      ("BCD", "2020-04-01", "411627"),
      ("BCD", "2020-04-02", "411626"),
      ("BCD", "2020-04-02", "411627"),
      ("DEF", "2019-01-09", "411626"))
      .toDF("user_id", "start_time", "end_time")
   df.groupBy(col("user_id"), col("start_time"))
      .agg(count(col("end_time")), sum(col("end_time")))
      .show()
+-------+----------+---------------+-------------+
|user_id|start_time|count(end_time)|sum(end_time)|
+-------+----------+---------------+-------------+
|    BCD|2020-04-02|              2|     823253.0|
|    ABC|2019-02-10|              2|     823253.0|
|    BCD|2020-04-01|              2|     823253.0|
|    DEF|2019-01-09|              1|     411626.0|
+-------+----------+---------------+-------------+

2. spark sql 使用 partition by

    df.withColumn("rank",row_number().over(Window.partitionBy(col("user_id"), col("start_time")).orderBy(col("end_time"))))
    .show()
+-------+----------+--------+----+
|user_id|start_time|end_time|rank|
+-------+----------+--------+----+
|    BCD|2020-04-02|  411626|   1|
|    BCD|2020-04-02|  411627|   2|
|    ABC|2019-02-10|  411626|   1|
|    ABC|2019-02-10|  411627|   2|
|    BCD|2020-04-01|  411626|   1|
|    BCD|2020-04-01|  411627|   2|
|    DEF|2019-01-09|  411626|   1|
+-------+----------+--------+----+

partition by 返所有数据列

3. group by实现返所有数据列

   df.groupBy(col("user_id"), col("start_time"))
      .agg(count(col("end_time")), sum(col("end_time")), collect_set(col("end_time"))(0).as("end_time"))
      .show()
+-------+----------+---------------+-------------+--------+
|user_id|start_time|count(end_time)|sum(end_time)|end_time|
+-------+----------+---------------+-------------+--------+
|    BCD|2020-04-02|              2|     823253.0|  411627|
|    ABC|2019-02-10|              2|     823253.0|  411627|
|    BCD|2020-04-01|              2|     823253.0|  411627|
|    DEF|2019-01-09|              1|     411626.0|  411626|
+-------+----------+---------------+-------------+--------+

使用 collect_set（去重）可以实现返回所有列