[SPARK-14614] [SQL] Add `bround` function

## What changes were proposed in this pull request?

This PR aims to add `bound` function (aka Banker's round) by extending current `round` implementation. [Hive supports `bround` since 1.3.0.](https://cwiki.apache.org/confluence/display/Hive/LanguageManual+UDF)

**Hive (1.3 ~ 2.0)**
```
hive> select round(2.5), bround(2.5);
OK
3.0	2.0
```

**After this PR**
```scala
scala> sql("select round(2.5), bround(2.5)").head
res0: org.apache.spark.sql.Row = [3,2]
```

## How was this patch tested?

Pass the Jenkins tests (with extended tests).

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #12376 from dongjoon-hyun/SPARK-14614.

2 files changed, 28 insertions, 1 deletions

diff --git a/sql/core/src/main/scala/org/apache/spark/sql/functions.scala b/sql/core/src/main/scala/org/apache/spark/sql/functions.scala
index 223122300d..8e2e94669b 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/functions.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/functions.scala
@@ -1777,6 +1777,23 @@ object functions {
   def round(e: Column, scale: Int): Column = withExpr { Round(e.expr, Literal(scale)) }
 
   /**
+   * Returns the value of the column `e` rounded to 0 decimal places with HALF_EVEN round mode.
+   *
+   * @group math_funcs
+   * @since 2.0.0
+   */
+  def bround(e: Column): Column = bround(e, 0)
+
+  /**
+   * Round the value of `e` to `scale` decimal places with HALF_EVEN round mode
+   * if `scale` >= 0 or at integral part when `scale` < 0.
+   *
+   * @group math_funcs
+   * @since 2.0.0
+   */
+  def bround(e: Column, scale: Int): Column = withExpr { BRound(e.expr, Literal(scale)) }
+
+  /**
    * Shift the given value numBits left. If the given value is a long value, this function
    * will return a long value else it will return an integer value.
    *
diff --git a/sql/core/src/test/scala/org/apache/spark/sql/MathExpressionsSuite.scala b/sql/core/src/test/scala/org/apache/spark/sql/MathExpressionsSuite.scala
index f5a67fd782..0de7f2321f 100644
--- a/sql/core/src/test/scala/org/apache/spark/sql/MathExpressionsSuite.scala
+++ b/sql/core/src/test/scala/org/apache/spark/sql/MathExpressionsSuite.scala
@@ -207,12 +207,16 @@ class MathExpressionsSuite extends QueryTest with SharedSQLContext {
     testOneToOneMathFunction(rint, math.rint)
   }
 
-  test("round") {
+  test("round/bround") {
     val df = Seq(5, 55, 555).map(Tuple1(_)).toDF("a")
     checkAnswer(
       df.select(round('a), round('a, -1), round('a, -2)),
       Seq(Row(5, 10, 0), Row(55, 60, 100), Row(555, 560, 600))
     )
+    checkAnswer(
+      df.select(bround('a), bround('a, -1), bround('a, -2)),
+      Seq(Row(5, 0, 0), Row(55, 60, 100), Row(555, 560, 600))
+    )
 
     val pi = "3.1415"
     checkAnswer(
@@ -221,6 +225,12 @@ class MathExpressionsSuite extends QueryTest with SharedSQLContext {
       Seq(Row(BigDecimal("0E3"), BigDecimal("0E2"), BigDecimal("0E1"), BigDecimal(3),
         BigDecimal("3.1"), BigDecimal("3.14"), BigDecimal("3.142")))
     )
+    checkAnswer(
+      sql(s"SELECT bround($pi, -3), bround($pi, -2), bround($pi, -1), " +
+        s"bround($pi, 0), bround($pi, 1), bround($pi, 2), bround($pi, 3)"),
+      Seq(Row(BigDecimal("0E3"), BigDecimal("0E2"), BigDecimal("0E1"), BigDecimal(3),
+        BigDecimal("3.1"), BigDecimal("3.14"), BigDecimal("3.142")))
+    )
   }
 
   test("exp") {