How to select the first row of each group?

Question

I have a DataFrame generated as follow:

df.groupBy($\"Hour\", $\"Category\")
  .agg(sum($\"value\") as \"TotalValue\")
  .sort($\"Hour\".asc, $\"TotalValue\"         


        

Answer 1

We can use the rank() window function (where you would choose the rank = 1) rank just adds a number for every row of a group (in this case it would be the hour)

here's an example. ( from https://github.com/jaceklaskowski/mastering-apache-spark-book/blob/master/spark-sql-functions.adoc#rank )

val dataset = spark.range(9).withColumn("bucket", 'id % 3)

import org.apache.spark.sql.expressions.Window
val byBucket = Window.partitionBy('bucket).orderBy('id)

scala> dataset.withColumn("rank", rank over byBucket).show
+---+------+----+
| id|bucket|rank|
+---+------+----+
|  0|     0|   1|
|  3|     0|   2|
|  6|     0|   3|
|  1|     1|   1|
|  4|     1|   2|
|  7|     1|   3|
|  2|     2|   1|
|  5|     2|   2|
|  8|     2|   3|
+---+------+----+