Expose Pivot() method in RelationalGroupedDataset (#598)

Author: moredatapls

src/csharp/Microsoft.Spark.E2ETest/IpcTests/Sql/DataFrameTests.cs

@@ -673,6 +673,14 @@ public void TestSignaturesV2_4_X()
             _df.IntersectAll(_df);
 
             _df.ExceptAll(_df);
+
+            {
+                RelationalGroupedDataset df = _df.GroupBy("name");
+
+                Assert.IsType<RelationalGroupedDataset>(df.Pivot("age"));
+
+                Assert.IsType<RelationalGroupedDataset>(df.Pivot(Col("age")));
+            }
         }
     }
 }

src/csharp/Microsoft.Spark/Sql/RelationalGroupedDataset.cs

@@ -85,6 +85,24 @@ public DataFrame Min(params string[] colNames) =>
         public DataFrame Sum(params string[] colNames) =>
             new DataFrame((JvmObjectReference)_jvmObject.Invoke("sum", (object)colNames));
 
+        /// <summary>
+        /// Pivots a column of the current DataFrame and performs the specified aggregation.
+        /// </summary>
+        /// <param name="pivotColumn">Name of the column to pivot</param>
+        /// <returns>New RelationalGroupedDataset object with pivot applied</returns>
+        public RelationalGroupedDataset Pivot(string pivotColumn) => 
+            new RelationalGroupedDataset(
+                (JvmObjectReference)_jvmObject.Invoke("pivot", pivotColumn), _dataFrame);
+
+        /// <summary>
+        /// Pivots a column of the current DataFrame and performs the specified aggregation.
+        /// </summary>
+        /// <param name="pivotColumn">The column to pivot</param>
+        /// <returns>New RelationalGroupedDataset object with pivot applied</returns>
+        public RelationalGroupedDataset Pivot(Column pivotColumn) => 
+            new RelationalGroupedDataset(
+                (JvmObjectReference)_jvmObject.Invoke("pivot", pivotColumn), _dataFrame);
+
         internal DataFrame Apply(StructType returnType, Func<FxDataFrame, FxDataFrame> func)
         {
             DataFrameGroupedMapWorkerFunction.ExecuteDelegate wrapper =