Github user fhueske commented on a diff in the pull request:
https://github.com/apache/flink/pull/2562#discussion_r81850803
--- Diff:
flink-libraries/flink-table/src/main/scala/org/apache/flink/api/table/windows.scala
---
@@ -0,0 +1,340 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.flink.api.table
+
+import org.apache.flink.api.table.expressions.{Expression,
ExpressionParser}
+import org.apache.flink.api.table.plan.logical._
+
+/**
+ * Group-window specification. Group-windows allow aggregates which are
computed for a group of
+ * elements. A (time or row-count) window is required to bound the
infinite input stream into a
+ * finite group. Group-windows are evaluated once per group.
+ */
+trait GroupWindow {
+
+ /**
+ * Converts an API class to a logical window for planning. This is an
internal method.
+ */
+ private[flink] def toLogicalWindow: LogicalWindow
+}
+
+/**
+ * A group-window operating on event-time.
+ *
+ * @param timeField defines the time mode for streaming tables and acts
as a time attribute for
+ * batch tables over which the query is evaluated.
+ */
+abstract class EventTimeWindow(timeField: Expression) extends GroupWindow {
+
+ protected var name: Option[Expression] = None
+
+ /**
+ * Assigns an alias for this window that the following `select()`
clause can refer to in order
+ * to access window properties such as window start or end time.
+ *
+ * @param alias alias for this window
+ * @return this window
+ */
+ def as(alias: Expression): EventTimeWindow = {
+ this.name = Some(alias)
+ this
+ }
+
+ /**
+ * Assigns an alias for this window that the following `select()`
clause can refer to in order
+ * to access window properties such as window start or end time.
+ *
+ * @param alias alias for this window
+ * @return this window
+ */
+ def as(alias: String): EventTimeWindow =
as(ExpressionParser.parseExpression(alias))
+}
+
+//
------------------------------------------------------------------------------------------------
+// Tumbling group-windows
+//
------------------------------------------------------------------------------------------------
+
+/**
+ * Tumbling group-window. By default, it works on processing-time.
--- End diff --
Add hint for how to create event time window.
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