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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>3.5. Window Functions</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets V1.79.1" /><link rel="prev" href="tutorial-transactions.html" title="3.4. Transactions" /><link rel="next" href="tutorial-inheritance.html" title="3.6. Inheritance" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">3.5. Window Functions</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="tutorial-transactions.html" title="3.4. Transactions">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="tutorial-advanced.html" title="Chapter 3. Advanced Features">Up</a></td><th width="60%" align="center">Chapter 3. Advanced Features</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 12.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="tutorial-inheritance.html" title="3.6. Inheritance">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="TUTORIAL-WINDOW"><div class="titlepage"><div><div><h2 class="title" style="clear: both">3.5. Window Functions</h2></div></div></div><a id="id-1.4.5.6.2" class="indexterm"></a><p>
    A <em class="firstterm">window function</em> performs a calculation across a set of
    table rows that are somehow related to the current row.  This is comparable
    to the type of calculation that can be done with an aggregate function.
    However, window functions do not cause rows to become grouped into a single
    output row like non-window aggregate calls would.  Instead, the
    rows retain their separate identities.  Behind the scenes, the window
    function is able to access more than just the current row of the query
    result.
   </p><p>
    Here is an example that shows how to compare each employee's salary
    with the average salary in his or her department:

</p><pre class="programlisting">
SELECT depname, empno, salary, avg(salary) OVER (PARTITION BY depname) FROM empsalary;
</pre><p>

</p><pre class="screen">
  depname  | empno | salary |          avg          
-----------+-------+--------+-----------------------
 develop   |    11 |   5200 | 5020.0000000000000000
 develop   |     7 |   4200 | 5020.0000000000000000
 develop   |     9 |   4500 | 5020.0000000000000000
 develop   |     8 |   6000 | 5020.0000000000000000
 develop   |    10 |   5200 | 5020.0000000000000000
 personnel |     5 |   3500 | 3700.0000000000000000
 personnel |     2 |   3900 | 3700.0000000000000000
 sales     |     3 |   4800 | 4866.6666666666666667
 sales     |     1 |   5000 | 4866.6666666666666667
 sales     |     4 |   4800 | 4866.6666666666666667
(10 rows)
</pre><p>

    The first three output columns come directly from the table
    <code class="structname">empsalary</code>, and there is one output row for each row in the
    table.  The fourth column represents an average taken across all the table
    rows that have the same <code class="structfield">depname</code> value as the current row.
    (This actually is the same function as the non-window <code class="function">avg</code>
    aggregate, but the <code class="literal">OVER</code> clause causes it to be
    treated as a window function and computed across the window frame.)
   </p><p>
    A window function call always contains an <code class="literal">OVER</code> clause
    directly following the window function's name and argument(s).  This is what
    syntactically distinguishes it from a normal function or non-window
    aggregate.  The <code class="literal">OVER</code> clause determines exactly how the
    rows of the query are split up for processing by the window function.
    The <code class="literal">PARTITION BY</code> clause within <code class="literal">OVER</code>
    divides the rows into groups, or partitions, that share the same
    values of the <code class="literal">PARTITION BY</code> expression(s).  For each row,
    the window function is computed across the rows that fall into the
    same partition as the current row.
   </p><p>
    You can also control the order in which rows are processed by
    window functions using <code class="literal">ORDER BY</code> within <code class="literal">OVER</code>.
    (The window <code class="literal">ORDER BY</code> does not even have to match the
    order in which the rows are output.)  Here is an example:

</p><pre class="programlisting">
SELECT depname, empno, salary,
       rank() OVER (PARTITION BY depname ORDER BY salary DESC)
FROM empsalary;
</pre><p>

</p><pre class="screen">
  depname  | empno | salary | rank 
-----------+-------+--------+------
 develop   |     8 |   6000 |    1
 develop   |    10 |   5200 |    2
 develop   |    11 |   5200 |    2
 develop   |     9 |   4500 |    4
 develop   |     7 |   4200 |    5
 personnel |     2 |   3900 |    1
 personnel |     5 |   3500 |    2
 sales     |     1 |   5000 |    1
 sales     |     4 |   4800 |    2
 sales     |     3 |   4800 |    2
(10 rows)
</pre><p>

    As shown here, the <code class="function">rank</code> function produces a numerical rank
    for each distinct <code class="literal">ORDER BY</code> value in the current row's
    partition, using the order defined by the <code class="literal">ORDER BY</code> clause.
    <code class="function">rank</code> needs no explicit parameter, because its behavior
    is entirely determined by the <code class="literal">OVER</code> clause.
   </p><p>
    The rows considered by a window function are those of the <span class="quote">“<span class="quote">virtual
    table</span>”</span> produced by the query's <code class="literal">FROM</code> clause as filtered by its
    <code class="literal">WHERE</code>, <code class="literal">GROUP BY</code>, and <code class="literal">HAVING</code> clauses
    if any.  For example, a row removed because it does not meet the
    <code class="literal">WHERE</code> condition is not seen by any window function.
    A query can contain multiple window functions that slice up the data
    in different ways using different <code class="literal">OVER</code> clauses, but
    they all act on the same collection of rows defined by this virtual table.
   </p><p>
    We already saw that <code class="literal">ORDER BY</code> can be omitted if the ordering
    of rows is not important.  It is also possible to omit <code class="literal">PARTITION
    BY</code>, in which case there is a single partition containing all rows.
   </p><p>
    There is another important concept associated with window functions:
    for each row, there is a set of rows within its partition called its
    <em class="firstterm">window frame</em>.  Some window functions act only
    on the rows of the window frame, rather than of the whole partition.
    By default, if <code class="literal">ORDER BY</code> is supplied then the frame consists of
    all rows from the start of the partition up through the current row, plus
    any following rows that are equal to the current row according to the
    <code class="literal">ORDER BY</code> clause.  When <code class="literal">ORDER BY</code> is omitted the
    default frame consists of all rows in the partition.
     <a href="#ftn.id-1.4.5.6.9.5" class="footnote"><sup class="footnote" id="id-1.4.5.6.9.5">[4]</sup></a>
    Here is an example using <code class="function">sum</code>:
   </p><pre class="programlisting">
SELECT salary, sum(salary) OVER () FROM empsalary;
</pre><pre class="screen">
 salary |  sum  
--------+-------
   5200 | 47100
   5000 | 47100
   3500 | 47100
   4800 | 47100
   3900 | 47100
   4200 | 47100
   4500 | 47100
   4800 | 47100
   6000 | 47100
   5200 | 47100
(10 rows)
</pre><p>
    Above, since there is no <code class="literal">ORDER BY</code> in the <code class="literal">OVER</code>
    clause, the window frame is the same as the partition, which for lack of
    <code class="literal">PARTITION BY</code> is the whole table; in other words each sum is
    taken over the whole table and so we get the same result for each output
    row.  But if we add an <code class="literal">ORDER BY</code> clause, we get very different
    results:
   </p><pre class="programlisting">
SELECT salary, sum(salary) OVER (ORDER BY salary) FROM empsalary;
</pre><pre class="screen">
 salary |  sum  
--------+-------
   3500 |  3500
   3900 |  7400
   4200 | 11600
   4500 | 16100
   4800 | 25700
   4800 | 25700
   5000 | 30700
   5200 | 41100
   5200 | 41100
   6000 | 47100
(10 rows)
</pre><p>
    Here the sum is taken from the first (lowest) salary up through the
    current one, including any duplicates of the current one (notice the
    results for the duplicated salaries).
   </p><p>
    Window functions are permitted only in the <code class="literal">SELECT</code> list
    and the <code class="literal">ORDER BY</code> clause of the query. They are forbidden
    elsewhere, such as in <code class="literal">GROUP BY</code>, <code class="literal">HAVING</code>
    and <code class="literal">WHERE</code> clauses.  This is because they logically
    execute after the processing of those clauses.  Also, window functions
    execute after non-window aggregate functions.  This means it is valid to
    include an aggregate function call in the arguments of a window function,
    but not vice versa.
   </p><p>
    If there is a need to filter or group rows after the window calculations
    are performed, you can use a sub-select.  For example:

</p><pre class="programlisting">
SELECT depname, empno, salary, enroll_date
FROM
  (SELECT depname, empno, salary, enroll_date,
          rank() OVER (PARTITION BY depname ORDER BY salary DESC, empno) AS pos
     FROM empsalary
  ) AS ss
WHERE pos &lt; 3;
</pre><p>

    The above query only shows the rows from the inner query having
    <code class="literal">rank</code> less than 3.
   </p><p>
    When a query involves multiple window functions, it is possible to write
    out each one with a separate <code class="literal">OVER</code> clause, but this is
    duplicative and error-prone if the same windowing behavior is wanted
    for several functions.  Instead, each windowing behavior can be named
    in a <code class="literal">WINDOW</code> clause and then referenced in <code class="literal">OVER</code>.
    For example:

</p><pre class="programlisting">
SELECT sum(salary) OVER w, avg(salary) OVER w
  FROM empsalary
  WINDOW w AS (PARTITION BY depname ORDER BY salary DESC);
</pre><p>
   </p><p>
    More details about window functions can be found in
    <a class="xref" href="sql-expressions.html#SYNTAX-WINDOW-FUNCTIONS" title="4.2.8. Window Function Calls">Section 4.2.8</a>,
    <a class="xref" href="functions-window.html" title="9.21. Window Functions">Section 9.21</a>,
    <a class="xref" href="queries-table-expressions.html#QUERIES-WINDOW" title="7.2.5. Window Function Processing">Section 7.2.5</a>, and the
    <a class="xref" href="sql-select.html" title="SELECT"><span class="refentrytitle">SELECT</span></a> reference page.
   </p><div class="footnotes"><br /><hr style="width:100; text-align:left;margin-left: 0" /><div id="ftn.id-1.4.5.6.9.5" class="footnote"><p><a href="#id-1.4.5.6.9.5" class="para"><sup class="para">[4] </sup></a>
       There are options to define the window frame in other ways, but
       this tutorial does not cover them.  See
       <a class="xref" href="sql-expressions.html#SYNTAX-WINDOW-FUNCTIONS" title="4.2.8. Window Function Calls">Section 4.2.8</a> for details.
      </p></div></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="tutorial-transactions.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="tutorial-advanced.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="tutorial-inheritance.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">3.4. Transactions </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 3.6. Inheritance</td></tr></table></div></body></html>