Project: Flink (App Definition and Development / Streaming & Messaging)
File: window-functions.md

title: "Window Functions" weight: 7 type: docs <!-- 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. --> Window functions are a kind of aggregation for a group of rows, referred as a window. It will return the aggregation value for each row based on the group of rows. ```sql window_function OVER ( [ { PARTITION | DISTRIBUTE } BY colName ( [, ... ] ) ] { ORDER | SORT } BY expression [ ASC | DESC ] [ NULLS { FIRST | LAST } ] [ , ... ] [ window_frame ] ) ``` Hive dialect supports the following window functions: Windowing functions LEAD LAG FIRST_VALUE LAST_VALUE {{< hint warning >}} Note: For FIRSTVALUE/LASTVALUE, use parameter to control skip null values or respect null values isn't supported yet. And they will always skip null values {{< /hint >}} Analytic functions RANK ROW_NUMBER DENSE_RANK CUME_DIST PERCENT_RANK NTILE Aggregate Functions COUNT SUM MIN MAX AVG It's used to specified which row to start on and where to end it. Window frame supports the following formats: ```sql (ROWS | RANGE) BETWEEN (UNBOUNDED | [num]) PRECEDING AND ([num] PRECEDING | CURRENT ROW | (UNBOUNDED | [num]) FOLLOWING) (ROWS | RANGE) BETWEEN CURRENT ROW AND (CURRENT ROW | (UNBOUNDED | [num]) FOLLOWING) (ROWS | RANGE) BETWEEN [num] FOLLOWING AND (UNBOUNDED | [num]) FOLLOWING ``` When `ORDER BY` is specified, but missing `window_frame`, the window frame defaults to `RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW`. When both `ORDER BY` and `window_frame` are missing, the window frame defaults to `ROW BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING`. {{< hint warning >}} Note: Distinct is not supported in window function yet. {{< /hint >}} ```sql -- PARTITION BY with one partitioning column, no ORDER BY or window specification SELECT a, COUNT(b) OVER (PARTITION BY c) FROM t; -- PARTITION BY with two partitioning columns, no ORDER BY or window specification SELECT a, COUNT(b) OVER (PARTITION BY c, d) FROM t; -- PARTITION BY with two partitioning columns, no ORDER BY or window specification SELECT a, SUM(b) OVER (PARTITION BY c, d ORDER BY e, f) FROM t; -- PARTITION BY with partitioning, ORDER BY, and window specification SELECT a, SUM(b) OVER (PARTITION BY c ORDER BY d ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) FROM t; SELECT a, AVG(b) OVER (PARTITION BY c ORDER BY d ROWS BETWEEN 3 PRECEDING AND CURRENT ROW) FROM t; SELECT a, AVG(b) OVER (PARTITION BY c ORDER BY d ROWS BETWEEN 3 PRECEDING AND 3 FOLLOWING) FROM t; SELECT a, AVG(b) OVER (PARTITION BY c ORDER BY d ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING) FROM t; ```