[SPARK-11325] [SQL] Alias 'alias' in Scala's DataFrame API

Author: Nong Li <nongli@gmail.com>

Closes #9286 from nongli/spark-11325.

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

@@ -698,6 +698,20 @@ class DataFrame private[sql](
    */
   def as(alias: Symbol): DataFrame = as(alias.name)
 
+  /**
+   * Returns a new [[DataFrame]] with an alias set. Same as `as`.
+   * @group dfops
+   * @since 1.6.0
+   */
+  def alias(alias: String): DataFrame = as(alias)
+
+  /**
+   * (Scala-specific) Returns a new [[DataFrame]] with an alias set. Same as `as`.
+   * @group dfops
+   * @since 1.6.0
+   */
+  def alias(alias: Symbol): DataFrame = as(alias)
+
   /**
    * Selects a set of column based expressions.
    * {{{

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

@@ -105,6 +105,13 @@ class DataFrameSuite extends QueryTest with SharedSQLContext {
     assert(testData.head(2).head.schema === testData.schema)
   }
 
+  test("dataframe alias") {
+    val df = Seq(Tuple1(1)).toDF("c").as("t")
+    val dfAlias = df.alias("t2")
+    df.col("t.c")
+    dfAlias.col("t2.c")
+  }
+
   test("simple explode") {
     val df = Seq(Tuple1("a b c"), Tuple1("d e")).toDF("words")