maropu commented on a change in pull request #28220: [SPARK-31390][SQL][DOCS]
Document Window Function in SQL Syntax Section
URL: https://github.com/apache/spark/pull/28220#discussion_r409381654
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File path: docs/sql-ref-syntax-qry-window.md
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@@ -19,4 +19,192 @@ license: |
limitations under the License.
---
-**This page is under construction**
+### Description
+
+Window functions operate on a group of rows, referred to as a window, and
calculate a return value for each row based on the group of rows. Window
functions are useful for processing tasks such as calculating a moving average,
computing a cumulative statistic, or accessing the value of rows given the
relative position of the current row.
+Spark SQL supports three types of window functions:
+
+ * Ranking Functions
+ * Analytic Functions
+ * Aggregate Functions
+
+### Syntax
+
+{% highlight sql %}
+window_function OVER
+( [ { PARTITION | DISTRIBUTE } BY partition_col_name = partition_col_val ( [
, ... ] ) ]
+ { ORDER | SORT } BY expression [ ASC | DESC ] [ NULLS { FIRST | LAST } ] [ ,
... ]
+ [ window_frame ] )
+{% endhighlight %}
+
+### Parameters
+
+<dl>
+ <dt><code><em>window_function</em></code></dt>
+ <dd>
+ <ul>
+ <li>Ranking Functions</li>
+ <br>
+ <b>Syntax:</b>
+ <code>
+ RANK | DENSE_RANK | PERCENT_RANK | NTILE | ROW_NUMBER
+ </code>
+ </ul>
+ <ul>
+ <li>Analytic Functions</li>
+ <br>
+ <b>Syntax:</b>
+ <code>
+ CUME_DIST | LAG | LEAD
+ </code>
+ </ul>
+ <ul>
+ <li>Aggregate Functions</li>
+ <br>
+ <b>Syntax:</b>
+ <code>
+ MAX | MIN | COUNT | SUM | AVG | ...
+ </code>
+ <br>
+ Please refer to the <a href="api/sql/">Built-in Functions</a> document
for a complete list of Spark aggregate functions.
+ </ul>
+ </dd>
+</dl>
+<dl>
+ <dt><code><em>window_frame</em></code></dt>
+ <dd>
+ Specifies which row to start the window on and where to end it.<br><br>
+ <b>Syntax:</b><br>
+ <code>
+ { RANGE | ROWS } [ BETWEEN ]
+ UNBOUNDED { PRECEDING | FOLLOWING }
+ | CURRENT ROW
+ | boolean_expression { PRECEDING | FOLLOWING }
+ </code> <br><br>
Review comment:
I think we need to describe what these clauses (RANGE, ROWS, BETWEEN, ...)
are.
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