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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 Vsnapshot" /><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 11.5 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 < 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.
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