techdocsmith commented on code in PR #14739: URL: https://github.com/apache/druid/pull/14739#discussion_r1282412053
########## docs/querying/sql-window-functions.md: ########## @@ -0,0 +1,164 @@ +--- +id: sql-window-functions +title: Window functions +--- + +<!-- + ~ 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. + --> + +> Apache Druid supports two query languages: [Druid SQL](sql.md) and [native queries](querying.md). +> This document describes the SQL language. +> +> Window functions are an [experimental](../development/experimental.md) feature. Development and testing are still at early stage. Feel free to try window functions and provide your feedback. +> There are known issues where ORDER BY only works on ascending order and certain options may cause errors. +> Set the context parameter `windowsAreForClosers: true` to use window functions. + +Window functions in Apache Druid produce values based upon the relationship of one row within a window of rows to the other rows within the same window. A window is a group of related rows within a result set. For example, rows with the same value for a specific dimension. + +The following example returns the rank of each row in ascending order based upon its `delta` value as related to other rows with the same `channel` value. + +```sql +SELECT FLOOR(__time TO DAY) AS event_time, + channel, + ABS(delta) AS change, + RANK() OVER w AS rank_value +FROM wikipedia +WHERE channel in ('#kk.wikipedia', '#lt.wikipedia') +AND '2016-06-28' > FLOOR(__time TO DAY) > '2016-06-26' +GROUP BY channel, ABS(delta), __time +WINDOW w AS (PARTITION BY channel ORDER BY abs(delta) ASC) +``` + +<details> +<summary> View results </summary> + +| event_time | channel | change | rank_value | +| -- | -- | -- | -- | +| 2016-06-27T00:00:00.000Z | #kk.wikipedia | 1 | 1 | +| 2016-06-27T00:00:00.000Z | #kk.wikipedia | 1 | 1 | +| 2016-06-27T00:00:00.000Z | #kk.wikipedia | 7 | 3 | +| 2016-06-27T00:00:00.000Z | #kk.wikipedia | 56 | 4 | +| 2016-06-27T00:00:00.000Z | #kk.wikipedia | 56 | 4 | +| 2016-06-27T00:00:00.000Z | #kk.wikipedia | 63 | 6 | +| 2016-06-27T00:00:00.000Z | #kk.wikipedia | 91 | 7 | +| 2016-06-27T00:00:00.000Z | #kk.wikipedia | 2440 | 8 | +| 2016-06-27T00:00:00.000Z | #kk.wikipedia | 2703 | 9 | +| 2016-06-27T00:00:00.000Z | #kk.wikipedia | 6900 |10 | +| 2016-06-27T00:00:00.000Z | #lt.wikipedia | 1 | 1 | +| 2016-06-27T00:00:00.000Z | #lt.wikipedia | 2 | 2 | +| 2016-06-27T00:00:00.000Z | #lt.wikipedia | 13 | 3 | +| 2016-06-27T00:00:00.000Z | #lt.wikipedia | 28 | 4 | +| 2016-06-27T00:00:00.000Z | #lt.wikipedia | 53 | 5 | +| 2016-06-27T00:00:00.000Z | #lt.wikipedia | 56 | 6 | +| 2016-06-27T00:00:00.000Z | #lt.wikipedia | 59 | 7 | +| 2016-06-27T00:00:00.000Z | #lt.wikipedia | 391 | 8 | +| 2016-06-27T00:00:00.000Z | #lt.wikipedia | 894 | 9 | +| 2016-06-27T00:00:00.000Z | #lt.wikipedia | 4358 | 10 | + +</details> + +Window functions are similar to [aggregation funtctions](./aggregations.md). + +You can use the OVER clause treat other Druid aggretation functions as window functions. For example, the sum of a value for rows within a window. + +When working with window functions, consider the following: +- Window functions only work on GROUP BY queries. +- Window functions support aliasing. + +## Define a window with the OVER clause + +The OVER clause defines the query windows for window functions as follows: +- PARTITION BY indicates the dimension that defines the rows within the window +- ORDER BY specifies the order of the rows within the windows. +Sometimes windows are called partitions, but don't confuse them with the segment partitioning feature of Druid. + + +The PARTITIONED BY clause defines the dimenson to use for the window. For example, the following OVER clause sets the window dimension to `channel` and orders the results by the absolute value of `delta` ascending: + +```sql +... +rank() OVER (PARTITION BY channel ORDER BY ABS(delta) ASC) +... +``` + +## Window function reference + +|Function|Notes|Default| Review Comment: Do we need it for the UI. 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