[SPARK-8026][SQL] Add Column.alias to Scala/Java DataFrame API

Author: Reynold Xin <rxin@databricks.com>

Closes #6565 from rxin/alias and squashes the following commits:

286d880 [Reynold Xin] [SPARK-8026][SQL] Add Column.alias to Scala/Java DataFrame API

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

@@ -716,6 +716,18 @@ class Column(protected[sql] val expr: Expression) extends Logging {
    */
   def endsWith(literal: String): Column = this.endsWith(lit(literal))
 
+  /**
+   * Gives the column an alias. Same as `as`.
+   * {{{
+   *   // Renames colA to colB in select output.
+   *   df.select($"colA".alias("colB"))
+   * }}}
+   *
+   * @group expr_ops
+   * @since 1.4.0
+   */
+  def alias(alias: String): Column = as(alias)
+
   /**
    * Gives the column an alias.
    * {{{

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

@@ -27,6 +27,12 @@ import org.apache.spark.sql.types._
 class ColumnExpressionSuite extends QueryTest {
   import org.apache.spark.sql.TestData._
 
+  test("alias") {
+    val df = Seq((1, Seq(1, 2, 3))).toDF("a", "intList")
+    assert(df.select(df("a").as("b")).columns.head === "b")
+    assert(df.select(df("a").alias("b")).columns.head === "b")
+  }
+
   test("single explode") {
     val df = Seq((1, Seq(1, 2, 3))).toDF("a", "intList")
     checkAnswer(