[GitHub] [spark] JoshRosen commented on a change in pull request #24557: [SPARK-27653][SQL] Add max_by() SQL aggregate function

[GitBox <gi...@apache.org>]

JoshRosen commented on a change in pull request #24557: [SPARK-27653][SQL] Add max_by() SQL aggregate function
URL: https://github.com/apache/spark/pull/24557#discussion_r282141704
 
 

 ##########
 File path: sql/core/src/test/scala/org/apache/spark/sql/DataFrameAggregateSuite.scala
 ##########
 @@ -782,4 +782,47 @@ class DataFrameAggregateSuite extends QueryTest with SharedSQLContext {
     val countAndDistinct = df.select(count("*"), countDistinct("*"))
     checkAnswer(countAndDistinct, Row(100000, 100))
   }
+
+  test("max_by") {
+    val yearOfMaxEarnings =
+      sql("SELECT course, max_by(year, earnings) FROM courseSales GROUP BY course")
+    checkAnswer(yearOfMaxEarnings, Row("dotNET", 2013) :: Row("Java", 2013) :: Nil)
+
+    checkAnswer(
+      sql("SELECT max_by(x, y) FROM VALUES (('a', 10)), (('b', 50)), (('c', 20)) AS tab(x, y)"),
+      Row("b") :: Nil
+    )
+
+    checkAnswer(
+      sql("SELECT max_by(x, y) FROM VALUES (('a', 10)), (('b', null)), (('c', 20)) AS tab(x, y)"),
+      Row("c") :: Nil
+    )
+
+    checkAnswer(
+      sql("SELECT max_by(x, y) FROM VALUES (('a', null)), (('b', null)), (('c', 20)) AS tab(x, y)"),
+      Row("c") :: Nil
+    )
+
+    checkAnswer(
+      sql("SELECT max_by(x, y) FROM VALUES (('a', 10)), (('b', 50)), (('c', null)) AS tab(x, y)"),
+      Row("b") :: Nil
+    )
+
+    checkAnswer(
+      sql("SELECT max_by(x, y) FROM VALUES (('a', null)), (('b', null)) AS tab(x, y)"),
 
 Review comment:
   This returns `null` because all values of the ordering column are `null`? That seems to match Presto behavior:
   
   ```
   SELECT max_by(x, y) FROM (
     VALUES
       ('a', null),
       ('b', null)
   ) AS tab (x, y)
   ```
   
   also returns `null` in Presto :thumbsup:

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