How to compare multiple rows?

Question

I\'d like to compare two consecutive rows i with i-1 of col2 (sorted by col1).

If item_i of the

Answer 1

You can use a combination of a window function and an aggregate to do this. The window function is used to get the next value of col2 (using col1 for ordering). The aggregate then counts the times we encounter a differences. This is implemented in the code below:

val data = Seq(
  ("row_1", "item_1"),
  ("row_2", "item_1"),
  ("row_3", "item_2"),
  ("row_4", "item_1"),
  ("row_5", "item_2"),
  ("row_6", "item_1")).toDF("col1", "col2")

import org.apache.spark.sql.expressions.Window
val q = data.
  withColumn("col2_next",
    coalesce(lead($"col2", 1) over Window.orderBy($"col1"), $"col2")).
  groupBy($"col2").
  agg(sum($"col2" =!= $"col2_next" cast "int") as "col3")

scala> q.show
17/08/22 10:15:53 WARN WindowExec: No Partition Defined for Window operation! Moving all data to a single partition, this can cause serious performance degradation.
+------+----+
|  col2|col3|
+------+----+
|item_1|   2|
|item_2|   2|
+------+----+