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    <h1 class="heading"><a href="../index.html">
        <span>Presto 0.104 Documentation</span></a></h1>
    <h2 class="heading"><span>5.13. Window Functions</span></h2>
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  <div class="section" id="window-functions">
<h1>5.13. Window Functions</h1>
<p>Window functions perform calculations across rows of the query result.
They run after the <tt class="docutils literal"><span class="pre">HAVING</span></tt> clause but before the <tt class="docutils literal"><span class="pre">ORDER</span> <span class="pre">BY</span></tt> clause.
Invoking a window function requires special syntax using the <tt class="docutils literal"><span class="pre">OVER</span></tt>
clause to specify the window. A window has three components:</p>
<ul class="simple">
<li>The partition specification, which separates the input rows into different
partitions. This is analogous to how the <tt class="docutils literal"><span class="pre">GROUP</span> <span class="pre">BY</span></tt> clause separates rows
into different groups for aggregate functions.</li>
<li>The ordering specification, which determines the order in which input rows
will be processed by the window function.</li>
<li>The window frame, which specifies a sliding window of rows to be processed
by the function for a given row. If the frame is not specified, it defaults
to <tt class="docutils literal"><span class="pre">RANGE</span> <span class="pre">UNBOUNDED</span> <span class="pre">PRECEDING</span></tt>, which is the same as
<tt class="docutils literal"><span class="pre">RANGE</span> <span class="pre">BETWEEN</span> <span class="pre">UNBOUNDED</span> <span class="pre">PRECEDING</span> <span class="pre">AND</span> <span class="pre">CURRENT</span> <span class="pre">ROW</span></tt>. This frame contains all
rows from the start of the partition up to the last peer of the current row.</li>
</ul>
<p>For example, the following query ranks orders for each clerk by price:</p>
<div class="highlight-sql"><div class="highlight"><pre><span class="k">SELECT</span> <span class="n">orderkey</span><span class="p">,</span> <span class="n">clerk</span><span class="p">,</span> <span class="n">totalprice</span><span class="p">,</span>
       <span class="n">rank</span><span class="p">()</span> <span class="n">OVER</span> <span class="p">(</span><span class="n">PARTITION</span> <span class="k">BY</span> <span class="n">clerk</span>
                    <span class="k">ORDER</span> <span class="k">BY</span> <span class="n">totalprice</span> <span class="k">DESC</span><span class="p">)</span> <span class="k">AS</span> <span class="n">rnk</span>
<span class="k">FROM</span> <span class="n">orders</span>
<span class="k">ORDER</span> <span class="k">BY</span> <span class="n">clerk</span><span class="p">,</span> <span class="n">rnk</span>
</pre></div>
</div>
<div class="section" id="aggregate-functions">
<h2>Aggregate Functions</h2>
<p>All <a class="reference internal" href="aggregate.html"><em>Aggregate Functions</em></a> can be used as window functions by adding the <tt class="docutils literal"><span class="pre">OVER</span></tt>
clause. The aggregate function is computed for each row over the rows within
the current row&#8217;s window frame.</p>
<p>For example, the following query produces a rolling sum of order prices
by day for each clerk:</p>
<div class="highlight-sql"><div class="highlight"><pre><span class="k">SELECT</span> <span class="n">clerk</span><span class="p">,</span> <span class="n">orderdate</span><span class="p">,</span> <span class="n">orderkey</span><span class="p">,</span> <span class="n">totalprice</span><span class="p">,</span>
       <span class="k">sum</span><span class="p">(</span><span class="n">totalprice</span><span class="p">)</span> <span class="n">OVER</span> <span class="p">(</span><span class="n">PARTITION</span> <span class="k">BY</span> <span class="n">clerk</span>
                             <span class="k">ORDER</span> <span class="k">BY</span> <span class="n">orderdate</span><span class="p">)</span> <span class="k">AS</span> <span class="n">rolling_sum</span>
<span class="k">FROM</span> <span class="n">orders</span>
<span class="k">ORDER</span> <span class="k">BY</span> <span class="n">clerk</span><span class="p">,</span> <span class="n">orderdate</span><span class="p">,</span> <span class="n">orderkey</span>
</pre></div>
</div>
</div>
<div class="section" id="ranking-functions">
<h2>Ranking Functions</h2>
<dl class="function">
<dt id="cume_dist">
<tt class="descname">cume_dist</tt><big>(</big><big>)</big> &rarr; bigint</dt>
<dd><p>Returns the cumulative distribution of a value in a group of values.
The result is the number of rows preceding or peer with the row in the
window ordering of the window partition divided by the total number of
rows in the window partition. Thus, any tie values in the ordering will
evaluate to the same distribution value.</p>
</dd></dl>

<dl class="function">
<dt id="dense_rank">
<tt class="descname">dense_rank</tt><big>(</big><big>)</big> &rarr; bigint</dt>
<dd><p>Returns the rank of a value in a group of values. This is similar to
<a class="reference internal" href="#rank" title="rank"><tt class="xref py py-func docutils literal"><span class="pre">rank()</span></tt></a>, except that tie values do not produce gaps in the sequence.</p>
</dd></dl>

<dl class="function">
<dt id="ntile">
<tt class="descname">ntile</tt><big>(</big><em>n</em><big>)</big> &rarr; bigint</dt>
<dd><p>Divides the rows for each window partition into <tt class="docutils literal"><span class="pre">n</span></tt> buckets ranging
from <tt class="docutils literal"><span class="pre">1</span></tt> to at most <tt class="docutils literal"><span class="pre">n</span></tt>. Bucket values will differ by at most <tt class="docutils literal"><span class="pre">1</span></tt>.
If the number of rows in the partition does not divide evenly into the
number of buckets, then the remainder values are distributed one per
bucket, starting with the first bucket.</p>
<p>For example, with <tt class="docutils literal"><span class="pre">6</span></tt> rows and <tt class="docutils literal"><span class="pre">4</span></tt> buckets, the bucket values would
be as follows: <tt class="docutils literal"><span class="pre">1</span></tt> <tt class="docutils literal"><span class="pre">1</span></tt> <tt class="docutils literal"><span class="pre">2</span></tt> <tt class="docutils literal"><span class="pre">2</span></tt> <tt class="docutils literal"><span class="pre">3</span></tt> <tt class="docutils literal"><span class="pre">4</span></tt></p>
</dd></dl>

<dl class="function">
<dt id="percent_rank">
<tt class="descname">percent_rank</tt><big>(</big><big>)</big> &rarr; bigint</dt>
<dd><p>Returns the percentage ranking of a value in group of values. The result
is <tt class="docutils literal"><span class="pre">(r</span> <span class="pre">-</span> <span class="pre">1)</span> <span class="pre">/</span> <span class="pre">(n</span> <span class="pre">-</span> <span class="pre">1)</span></tt> where <tt class="docutils literal"><span class="pre">r</span></tt> is the <a class="reference internal" href="#rank" title="rank"><tt class="xref py py-func docutils literal"><span class="pre">rank()</span></tt></a> of the row and
<tt class="docutils literal"><span class="pre">n</span></tt> is the total number of rows in the window partition.</p>
</dd></dl>

<dl class="function">
<dt id="rank">
<tt class="descname">rank</tt><big>(</big><big>)</big> &rarr; bigint</dt>
<dd><p>Returns the rank of a value in a group of values. The rank is one plus
the number of rows preceding the row that are not peer with the row.
Thus, tie values in the ordering will produce gaps in the sequence.
The ranking is performed for each window partition.</p>
</dd></dl>

<dl class="function">
<dt id="row_number">
<tt class="descname">row_number</tt><big>(</big><big>)</big> &rarr; bigint</dt>
<dd><p>Returns a unique, sequential number for each row, starting with one,
according to the ordering of rows within the window partition.</p>
</dd></dl>

</div>
<div class="section" id="value-functions">
<h2>Value Functions</h2>
<dl class="function">
<dt id="first_value">
<tt class="descname">first_value</tt><big>(</big><em>x</em><big>)</big> &rarr; [same as input]</dt>
<dd><p>Returns the first value of the window.</p>
</dd></dl>

<dl class="function">
<dt id="last_value">
<tt class="descname">last_value</tt><big>(</big><em>x</em><big>)</big> &rarr; [same as input]</dt>
<dd><p>Returns the last value of the window.</p>
</dd></dl>

<dl class="function">
<dt id="nth_value">
<tt class="descname">nth_value</tt><big>(</big><em>x</em>, <em>offset</em><big>)</big> &rarr; [same as input]</dt>
<dd><p>Returns the value at the specified offset from beginning the window.
Offsets start at <tt class="docutils literal"><span class="pre">1</span></tt>. The offset can be any scalar
expression.  If the offset is null or greater than the number of values in
the window, null is returned.  It is an error for the offset to be zero or
negative.</p>
</dd></dl>

<dl class="function">
<dt id="lead">
<tt class="descname">lead</tt><big>(</big><em>x</em><span class="optional">[</span>, <em>offset</em><span class="optional">[</span>, <em>default_value</em><span class="optional">]</span><span class="optional">]</span><big>)</big> &rarr; [same as input]</dt>
<dd><p>Returns the value at <tt class="docutils literal"><span class="pre">offset</span></tt> rows after the current row in the window.
Offsets start at <tt class="docutils literal"><span class="pre">0</span></tt>, which is the current row. The
offset can be any scalar expression.  The default <tt class="docutils literal"><span class="pre">offset</span></tt> is <tt class="docutils literal"><span class="pre">1</span></tt>. If the
offset is null or larger than the window, the <tt class="docutils literal"><span class="pre">default_value</span></tt> is returned,
or if it is not specified <tt class="docutils literal"><span class="pre">null</span></tt> is returned.</p>
</dd></dl>

<dl class="function">
<dt id="lag">
<tt class="descname">lag</tt><big>(</big><em>x</em><span class="optional">[</span>, <em>offset</em><span class="optional">[</span>, <em>default_value</em><span class="optional">]</span><span class="optional">]</span><big>)</big> &rarr; [same as input]</dt>
<dd><p>Returns the value at <tt class="docutils literal"><span class="pre">offset</span></tt> rows before the current row in the window
Offsets start at <tt class="docutils literal"><span class="pre">0</span></tt>, which is the current row.  The
offset can be any scalar expression.  The default <tt class="docutils literal"><span class="pre">offset</span></tt> is <tt class="docutils literal"><span class="pre">1</span></tt>. If the
offset is null or larger than the window, the <tt class="docutils literal"><span class="pre">default_value</span></tt> is returned,
or if it is not specified <tt class="docutils literal"><span class="pre">null</span></tt> is returned.</p>
</dd></dl>

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