GroupBy column and filter rows with maximum value in Pyspark

Question

I am almost certain this has been asked before, but a search through stackoverflow did not answer my question. Not a duplicate of [2] since I want the maximum value, not the most frequent item. I am new to pyspark and trying to do something really simple: I want to groupBy column "A" and then only keep the row of each group that has the maximum value in column "B". Like this:

df_cleaned = df.groupBy("A").agg(F.max("B"))

Unfortunately, this throws away all other columns - df_cleaned only contains the columns "A" and the max value of B. How do I instead keep the rows? ("A", "B", "C"...)

Answer 1

You can do this without a udf using a Window.

Consider the following example:

import pyspark.sql.functions as f
data = [
    ('a', 5),
    ('a', 8),
    ('a', 7),
    ('b', 1),
    ('b', 3)
]
df = sqlCtx.createDataFrame(data, ["A", "B"])
df.show()
#+---+---+
#|  A|  B|
#+---+---+
#|  a|  5|
#|  a|  8|
#|  a|  7|
#|  b|  1|
#|  b|  3|
#+---+---+

Create a Window to partition by column A and use this to compute the maximum of each group. Then filter out the rows such that the value in column B is equal to the max.

from pyspark.sql import Window
w = Window.partitionBy('A')
df.withColumn('maxB', f.max('B').over(w))\
    .where(f.col('B') == f.col('maxB'))\
    .drop('maxB')\
    .show()
#+---+---+
#|  A|  B|
#+---+---+
#|  a|  8|
#|  b|  3|
#+---+---+

Or equivalently using pyspark-sql:

df.registerTempTable('table')
q = "SELECT A, B FROM (SELECT *, MAX(B) OVER (PARTITION BY A) AS maxB FROM table) M WHERE B = maxB"
sqlCtx.sql(q).show()
#+---+---+
#|  A|  B|
#+---+---+
#|  b|  3|
#|  a|  8|
#+---+---+