[SPARK-7280][SQL] Add "drop" column/s on a data frame

Takes a column name/s and returns a new DataFrame that drops a column/s.

Author: rakeshchalasani <vnit.rakesh@gmail.com>

Closes #5818 from rakeshchalasani/SPARK-7280 and squashes the following commits:

ce2ec09 [rakeshchalasani] Minor edit
45c06f1 [rakeshchalasani] Change withColumnRename and format changes
f68945a [rakeshchalasani] Minor fix
0b9104d [rakeshchalasani] Drop one column at a time
289afd2 [rakeshchalasani] [SPARK-7280][SQL] Add "drop" column/s on a data frame

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

@@ -851,15 +851,40 @@ class DataFrame private[sql](
 
   /**
    * Returns a new [[DataFrame]] with a column renamed.
+   * This is a no-op if schema doesn't contain existingName.
    * @group dfops
    */
   def withColumnRenamed(existingName: String, newName: String): DataFrame = {
     val resolver = sqlContext.analyzer.resolver
+    val shouldRename = schema.exists(f => resolver(f.name, existingName))
+    if (shouldRename) {
       val colNames = schema.map { field =>
         val name = field.name
         if (resolver(name, existingName)) Column(name).as(newName) else Column(name)
       }
       select(colNames : _*)
+    } else {
+      this
+    }
+  }
+
+  /**
+   * Returns a new [[DataFrame]] with a column dropped.
+   * This is a no-op if schema doesn't contain column name.
+   * @group dfops
+   */
+  def drop(colName: String): DataFrame = {
+    val resolver = sqlContext.analyzer.resolver
+    val shouldDrop = schema.exists(f => resolver(f.name, colName))
+    if (shouldDrop) {
+      val colsAfterDrop = schema.filter { field =>
+        val name = field.name
+        !resolver(name, colName)
+      }.map(f => Column(f.name))
+      select(colsAfterDrop : _*)
+    } else {
+      this
+    }
   }
 
   /**

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

@@ -499,6 +499,22 @@ class DataFrameSuite extends QueryTest {
       Row(2) :: Row(3) :: Row(4) :: Nil)
   }
 
+  test("drop column using drop") {
+    val df = testData.drop("key")
+    checkAnswer(
+      df,
+      testData.collect().map(x => Row(x.getString(1))).toSeq)
+    assert(df.schema.map(_.name) === Seq("value"))
+  }
+
+  test("drop unknown column (no-op)") {
+    val df = testData.drop("random")
+    checkAnswer(
+      df,
+      testData.collect().toSeq)
+    assert(df.schema.map(_.name) === Seq("key","value"))
+  }
+
   test("withColumnRenamed") {
     val df = testData.toDF().withColumn("newCol", col("key") + 1)
       .withColumnRenamed("value", "valueRenamed")