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diff --git a/doc/src/sgml/html/tutorial-window.html b/doc/src/sgml/html/tutorial-window.html new file mode 100644 index 0000000..1459518 --- /dev/null +++ b/doc/src/sgml/html/tutorial-window.html @@ -0,0 +1,202 @@ +<?xml version="1.0" encoding="UTF-8" standalone="no"?> +<!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 id="docContent" class="container-fluid col-10"><div 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 15.5 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 /></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">[5]</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="9.22. Window Functions">Section 9.22</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">[5] </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" title="3.4. Transactions">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="tutorial-advanced.html" title="Chapter 3. 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