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Doc: simplify the tutorial's window-function examples.

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For the purposes of this discussion, row_number() is just as good
as rank(), and its behavior is easier to understand and describe.
So let's switch the examples to using row_number().

Along the way to checking the results given in the tutorial,
I found it helpful to extract the empsalary table we use in the
regression tests, which is evidently the same data that was used
to make these results.  So I shoved that into advanced.source
to improve the coverage of that file a little.  (There's still
several pages of the tutorial that are not included in it,
but at least now 3.5 Window Functions is covered.)

Suggested-by: "David G. Johnston" <david.g.johnston@gmail.com>
Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/173737973383.1070.1832752929070067441@wrigleys.postgresql.org
This commit is contained in:
Tom Lane
2025-01-21 14:43:21 -05:00
parent db19a5061c
commit 4907ba304c
2 changed files with 71 additions and 19 deletions
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41
doc/src/sgml/advanced.sgml
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@ -389,30 +389,32 @@ SELECT depname, empno, salary, avg(salary) OVER (PARTITION BY depname) FROM emps
<programlisting>
SELECT depname, empno, salary,
rank() OVER (PARTITION BY depname ORDER BY salary DESC)
row_number() OVER (PARTITION BY depname ORDER BY salary DESC)
FROM empsalary;
</programlisting>
<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
depname | empno | salary | row_number
-----------+-------+--------+------------
develop | 8 | 6000 | 1
develop | 10 | 5200 | 2
develop | 11 | 5200 | 3
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 | 3
(10 rows)
</screen>
As shown here, the <function>rank</function> function produces a numerical rank
for each distinct <literal>ORDER BY</literal> value in the current row's
partition, using the order defined by the <literal>ORDER BY</literal> clause.
<function>rank</function> needs no explicit parameter, because its behavior
As shown here, the <function>row_number</function> window function
assigns sequential numbers to the rows within each partition,
in the order defined by the <literal>ORDER BY</literal> clause
(with tied rows numbered in an unspecified order).
<function>row_number</function> needs no explicit parameter,
because its behavior
is entirely determined by the <literal>OVER</literal> clause.
</para>
@ -527,14 +529,15 @@ SELECT salary, sum(salary) OVER (ORDER BY salary) FROM empsalary;
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
row_number() OVER (PARTITION BY depname ORDER BY salary DESC, empno) AS pos
FROM empsalary
) AS ss
WHERE pos &lt; 3;
</programlisting>
The above query only shows the rows from the inner query having
<literal>rank</literal> less than 3.
<literal>row_number</literal> less than 3 (that is, the first
two rows for each department).
</para>
<para>