Avoiding duplicate coulmns after nullSafeJoin scala spark - scala

Question

I have use case wherein I need to join nullable columns. I am doing the same like this :
def nullSafeJoin(leftDF: DataFrame, rightDF: DataFrame, joinOnColumns: Seq[String]) = {
val dataset1 = leftDF.alias("dataset1")
val dataset2 = rightDF.alias("dataset2")
val firstColumn = joinOnColumns.head
val colExpression: Column = (col(s"dataset1.$firstColumn").eqNullSafe(col(s"dataset2.$firstColumn")))
val fullExpr = joinOnColumns.tail.foldLeft(colExpression) {
(colExpression, p) => colExpression && (col(s"dataset1.$p").eqNullSafe(col(s"dataset2.$p")))
}
dataset1.join(dataset2, fullExpr)
}
The final joined dataset has duplicate columns. I have tried dropping the columns using the alias like this :
dataset1.join(dataset2, fullExpr).drop(s"dataset2.$firstColumn")
but it doesn't work.
I understand that instead of dropping we can do a select columns.
I am trying to have a generic code base so don't want to pass the list of columns to be selected to the function (In case of drop I would be having to just drop the list of joinOnColumns we have passed to the function)
Any pointers on how to solve this would be really helpful.
Thanks!
Edit : (Sample data )
leftDF :
+------------------+-----------+---------+---------+-------+
| A| B| C| D| status|
+------------------+-----------+---------+---------+-------+
| 14567| 37| 1| game|Enabled|
| 14567| BASE| 1| toy| Paused|
| 13478| null| 5| game|Enabled|
| 2001| BASE| 1| null| Paused|
| null| 37| 1| home|Enabled|
+------------------+-----------+---------+---------+-------+
rightDF :
+------------------+-----------+---------+
| A| B| C|
+------------------+-----------+---------+
| 140| 37| 1|
| 569| BASE| 1|
| 13478| null| 5|
| 2001| BASE| 1|
| null| 37| 1|
+------------------+-----------+---------+
Final Join (Required):
+------------------+-----------+---------+---------+-------+
| A| B| C| D| status|
+------------------+-----------+---------+---------+-------+
| 13478| null| 5| game|Enabled|
| 2001| BASE| 1| null| Paused|
| null| 37| 1| home|Enabled|
+------------------+-----------+---------+---------+-------+

Answer 1

Your final DataFrame has duplicate columns from both leftDF & rightDF, don't have identifier to check if that column is from leftDF or rightDF.
So I have renamed leftDF & rightDF columns. leftDF columns starts with left_[column_name] & rightDF columns starts with right_[column_name]
Hope below code will help you.
scala> :paste
// Entering paste mode (ctrl-D to finish)
val left = Seq(("14567", "37", "1", "game", "Enabled"), ("14567", "BASE", "1", "toy", "Paused"), ("13478", "null", "5", "game", "Enabled"), ("2001", "BASE", "1", "null", "Paused"), ("null", "37", "1", "home", "Enabled")).toDF("a", "b", "c", "d", "status")
val right = Seq(("140", "37", 1), ("569", "BASE", 1), ("13478", "null", 5), ("2001", "BASE", 1), ("null", "37", 1)).toDF("a", "b", "c")
import org.apache.spark.sql.DataFrame
def nullSafeJoin(leftDF: DataFrame, rightDF: DataFrame, joinOnColumns: Seq[String]):DataFrame = {
val leftRenamedDF = leftDF
.columns
.map(c => (c, s"left_${c}"))
.foldLeft(leftDF){ (df, c) =>
df.withColumnRenamed(c._1, c._2)
}
val rightRenamedDF = rightDF
.columns
.map(c => (c, s"right_${c}"))
.foldLeft(rightDF){(df, c) =>
df.withColumnRenamed(c._1, c._2)
}
val fullExpr = joinOnColumns
.tail
.foldLeft($"left_${joinOnColumns.head}".eqNullSafe($"right_${joinOnColumns.head}")){(cee, p) =>
cee && ($"left_${p}".eqNullSafe($"right_${p}"))
}
val finalColumns = joinOnColumns
.map(c => col(s"left_${c}").as(c)) ++ // Taking All columns from Join columns
leftDF.columns.diff(joinOnColumns).map(c => col(s"left_${c}").as(c)) ++ // Taking missing columns from leftDF
rightDF.columns.diff(joinOnColumns).map(c => col(s"right_${c}").as(c)) // Taking missing columns from rightDF
leftRenamedDF.join(rightRenamedDF, fullExpr).select(finalColumns: _*)
}
scala>
Final DataFrame result is :
scala> nullSafeJoin(left, right, Seq("a", "b", "c")).show(false)
// Exiting paste mode, now interpreting.
+-----+----+---+----+-------+
|a |b |c |d |status |
+-----+----+---+----+-------+
|13478|null|5 |game|Enabled|
|2001 |BASE|1 |null|Paused |
|null |37 |1 |home|Enabled|
+-----+----+---+----+-------+