How to update column based on a condition (a value in a group)?

Question

I have the following df:

+---+----+-----+
|sno|dept|color|
+---+----+-----+
|  1|  fn|  red|
|  2|  fn| blue|
|  3|  fn|green|
+---+----+-----+

<

Answer 1

Given:

val df = Seq(
  (1, "fn", "red"),
  (2, "fn", "blue"),
  (3, "fn", "green"),
  (4, "aa", "blue"),
  (5, "aa", "green"),
  (6, "bb", "red"),
  (7, "bb", "red"),
  (8, "aa", "blue")
).toDF("id", "fn", "color")

Do the calculation:

val redOrNot = df.groupBy("fn")
  .agg(collect_set('color) as "values")
  .withColumn("hasRed", array_contains('values, "red"))

// gives null for no option
val colorPicker = when('hasRed, "red")
val result = df.join(redOrNot, "fn")
  .withColumn("resultColor", colorPicker) 
  .withColumn("color", coalesce('resultColor, 'color)) // skips nulls that leads to the answer
  .select('id, 'fn, 'color)

The result looks as follows (that seems to be an answer):

scala> result.show
+---+---+-----+
| id| fn|color|
+---+---+-----+
|  1| fn|  red|
|  2| fn|  red|
|  3| fn|  red|
|  4| aa| blue|
|  5| aa|green|
|  6| bb|  red|
|  7| bb|  red|
|  8| aa| blue|
+---+---+-----+

You can chain when operators and have a default value with otherwise. Consult the scaladoc of when operator.

I think you could do something very similar (and perhaps more efficient) using windowed operators or user-defined aggregate functions (UDAF), but...well...don't currently know how to do it. Leaving the comment here to inspire others ;-)

p.s. Learnt a lot! Thanks for the idea!