[SPARK-12011][SQL] Stddev/Variance etc should support columnName as arguments

Spark SQL aggregate function:
```Java
stddev
stddev_pop
stddev_samp
variance
var_pop
var_samp
skewness
kurtosis
collect_list
collect_set
```
should support ```columnName``` as arguments like other aggregate function(max/min/count/sum).

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #9994 from yanboliang/SPARK-12011.

sql/core/src/main/scala/org/apache/spark/sql/functions.scala

@@ -214,6 +214,16 @@ object functions extends LegacyFunctions {
    */
   def collect_list(e: Column): Column = callUDF("collect_list", e)
 
+  /**
+   * Aggregate function: returns a list of objects with duplicates.
+   *
+   * For now this is an alias for the collect_list Hive UDAF.
+   *
+   * @group agg_funcs
+   * @since 1.6.0
+   */
+  def collect_list(columnName: String): Column = collect_list(Column(columnName))
+
   /**
    * Aggregate function: returns a set of objects with duplicate elements eliminated.
    *
@@ -224,6 +234,16 @@ object functions extends LegacyFunctions {
    */
   def collect_set(e: Column): Column = callUDF("collect_set", e)
 
+  /**
+   * Aggregate function: returns a set of objects with duplicate elements eliminated.
+   *
+   * For now this is an alias for the collect_set Hive UDAF.
+   *
+   * @group agg_funcs
+   * @since 1.6.0
+   */
+  def collect_set(columnName: String): Column = collect_set(Column(columnName))
+
   /**
    * Aggregate function: returns the Pearson Correlation Coefficient for two columns.
    *
@@ -312,6 +332,14 @@ object functions extends LegacyFunctions {
    */
   def kurtosis(e: Column): Column = withAggregateFunction { Kurtosis(e.expr) }
 
+  /**
+   * Aggregate function: returns the kurtosis of the values in a group.
+   *
+   * @group agg_funcs
+   * @since 1.6.0
+   */
+  def kurtosis(columnName: String): Column = kurtosis(Column(columnName))
+
   /**
    * Aggregate function: returns the last value in a group.
    *
@@ -386,6 +414,14 @@ object functions extends LegacyFunctions {
    */
   def skewness(e: Column): Column = withAggregateFunction { Skewness(e.expr) }
 
+  /**
+   * Aggregate function: returns the skewness of the values in a group.
+   *
+   * @group agg_funcs
+   * @since 1.6.0
+   */
+  def skewness(columnName: String): Column = skewness(Column(columnName))
+
   /**
    * Aggregate function: alias for [[stddev_samp]].
    *
@@ -394,6 +430,14 @@ object functions extends LegacyFunctions {
    */
   def stddev(e: Column): Column = withAggregateFunction { StddevSamp(e.expr) }
 
+  /**
+   * Aggregate function: alias for [[stddev_samp]].
+   *
+   * @group agg_funcs
+   * @since 1.6.0
+   */
+  def stddev(columnName: String): Column = stddev(Column(columnName))
+
   /**
    * Aggregate function: returns the sample standard deviation of
    * the expression in a group.
@@ -403,6 +447,15 @@ object functions extends LegacyFunctions {
    */
   def stddev_samp(e: Column): Column = withAggregateFunction { StddevSamp(e.expr) }
 
+  /**
+   * Aggregate function: returns the sample standard deviation of
+   * the expression in a group.
+   *
+   * @group agg_funcs
+   * @since 1.6.0
+   */
+  def stddev_samp(columnName: String): Column = stddev_samp(Column(columnName))
+
   /**
    * Aggregate function: returns the population standard deviation of
    * the expression in a group.
@@ -412,6 +465,15 @@ object functions extends LegacyFunctions {
    */
   def stddev_pop(e: Column): Column = withAggregateFunction { StddevPop(e.expr) }
 
+  /**
+   * Aggregate function: returns the population standard deviation of
+   * the expression in a group.
+   *
+   * @group agg_funcs
+   * @since 1.6.0
+   */
+  def stddev_pop(columnName: String): Column = stddev_pop(Column(columnName))
+
   /**
    * Aggregate function: returns the sum of all values in the expression.
    *
@@ -452,6 +514,14 @@ object functions extends LegacyFunctions {
    */
   def variance(e: Column): Column = withAggregateFunction { VarianceSamp(e.expr) }
 
+  /**
+   * Aggregate function: alias for [[var_samp]].
+   *
+   * @group agg_funcs
+   * @since 1.6.0
+   */
+  def variance(columnName: String): Column = variance(Column(columnName))
+
   /**
    * Aggregate function: returns the unbiased variance of the values in a group.
    *
@@ -460,6 +530,14 @@ object functions extends LegacyFunctions {
    */
   def var_samp(e: Column): Column = withAggregateFunction { VarianceSamp(e.expr) }
 
+  /**
+   * Aggregate function: returns the unbiased variance of the values in a group.
+   *
+   * @group agg_funcs
+   * @since 1.6.0
+   */
+  def var_samp(columnName: String): Column = var_samp(Column(columnName))
+
   /**
    * Aggregate function: returns the population variance of the values in a group.
    *
@@ -468,6 +546,14 @@ object functions extends LegacyFunctions {
    */
   def var_pop(e: Column): Column = withAggregateFunction { VariancePop(e.expr) }
 
+  /**
+   * Aggregate function: returns the population variance of the values in a group.
+   *
+   * @group agg_funcs
+   * @since 1.6.0
+   */
+  def var_pop(columnName: String): Column = var_pop(Column(columnName))
+
   //////////////////////////////////////////////////////////////////////////////////////////////
   // Window functions
   //////////////////////////////////////////////////////////////////////////////////////////////

sql/core/src/test/scala/org/apache/spark/sql/DataFrameAggregateSuite.scala

@@ -261,6 +261,9 @@ class DataFrameAggregateSuite extends QueryTest with SharedSQLContext {
     checkAnswer(
       testData2.agg(stddev('a), stddev_pop('a), stddev_samp('a)),
       Row(testData2ADev, math.sqrt(4 / 6.0), testData2ADev))
+    checkAnswer(
+      testData2.agg(stddev("a"), stddev_pop("a"), stddev_samp("a")),
+      Row(testData2ADev, math.sqrt(4 / 6.0), testData2ADev))
   }
 
   test("zero stddev") {