[SPARK-10747][SQL] Support NULLS FIRST|LAST clause in ORDER BY

## What changes were proposed in this pull request?
Currently, ORDER BY clause returns nulls value according to sorting order (ASC|DESC), considering null value is always smaller than non-null values.
However, SQL2003 standard support NULLS FIRST or NULLS LAST to allow users to specify whether null values should be returned first or last, regardless of sorting order (ASC|DESC).

This PR is to support this new feature.

## How was this patch tested?
New test cases are added to test NULLS FIRST|LAST for regular select queries and windowing queries.

(If this patch involves UI changes, please attach a screenshot; otherwise, remove this)

Author: Xin Wu <xinwu@us.ibm.com>

Closes #14842 from xwu0226/SPARK-10747.

1 files changed, 2 insertions, 1 deletions

diff --git a/sql/core/src/test/scala/org/apache/spark/sql/execution/SortSuite.scala b/sql/core/src/test/scala/org/apache/spark/sql/execution/SortSuite.scala
index ba3fa3732d..a7bbe34f4e 100644
--- a/sql/core/src/test/scala/org/apache/spark/sql/execution/SortSuite.scala
+++ b/sql/core/src/test/scala/org/apache/spark/sql/execution/SortSuite.scala
@@ -101,7 +101,8 @@ class SortSuite extends SparkPlanTest with SharedSQLContext {
   for (
     dataType <- DataTypeTestUtils.atomicTypes ++ Set(NullType);
     nullable <- Seq(true, false);
-    sortOrder <- Seq('a.asc :: Nil, 'a.desc :: Nil);
+    sortOrder <-
+      Seq('a.asc :: Nil, 'a.asc_nullsLast :: Nil, 'a.desc :: Nil, 'a.desc_nullsFirst :: Nil);
     randomDataGenerator <- RandomDataGenerator.forType(dataType, nullable)
   ) {
     test(s"sorting on $dataType with nullable=$nullable, sortOrder=$sortOrder") {