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><H1
CLASS="SECT1"
><A
NAME="TUTORIAL-WINDOW"
>3.5. Window Functions</A
></H1
><P
> A <I
CLASS="FIRSTTERM"
>window function</I
> 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.
But unlike regular aggregate functions, use of a window function does not
cause rows to become grouped into a single output row — 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
<TT
CLASS="STRUCTNAME"
>empsalary</TT
>, 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 <TT
CLASS="STRUCTFIELD"
>depname</TT
> value as the current row.
(This actually is the same function as the regular <CODE
CLASS="FUNCTION"
>avg</CODE
>
aggregate function, but the <TT
CLASS="LITERAL"
>OVER</TT
> clause causes it to be
treated as a window function and computed across an appropriate set of
rows.)
</P
><P
> A window function call always contains an <TT
CLASS="LITERAL"
>OVER</TT
> clause
directly following the window function's name and argument(s). This is what
syntactically distinguishes it from a regular function or aggregate
function. The <TT
CLASS="LITERAL"
>OVER</TT
> clause determines exactly how the
rows of the query are split up for processing by the window function.
The <TT
CLASS="LITERAL"
>PARTITION BY</TT
> list within <TT
CLASS="LITERAL"
>OVER</TT
> specifies
dividing the rows into groups, or partitions, that share the same
values of the <TT
CLASS="LITERAL"
>PARTITION BY</TT
> 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 <TT
CLASS="LITERAL"
>ORDER BY</TT
> within <TT
CLASS="LITERAL"
>OVER</TT
>.
(The window <TT
CLASS="LITERAL"
>ORDER BY</TT
> 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
within the current row's partition for each distinct <TT
CLASS="LITERAL"
>ORDER BY</TT
>
value, in the order defined by the <TT
CLASS="LITERAL"
>ORDER BY</TT
> clause.
<CODE
CLASS="FUNCTION"
>rank</CODE
> needs no explicit parameter, because its behavior
is entirely determined by the <TT
CLASS="LITERAL"
>OVER</TT
> clause.
</P
><P
> The rows considered by a window function are those of the <SPAN
CLASS="QUOTE"
>"virtual
table"</SPAN
> produced by the query's <TT
CLASS="LITERAL"
>FROM</TT
> clause as filtered by its
<TT
CLASS="LITERAL"
>WHERE</TT
>, <TT
CLASS="LITERAL"
>GROUP BY</TT
>, and <TT
CLASS="LITERAL"
>HAVING</TT
> clauses
if any. For example, a row removed because it does not meet the
<TT
CLASS="LITERAL"
>WHERE</TT
> condition is not seen by any window function.
A query can contain multiple window functions that slice up the data
in different ways by means of different <TT
CLASS="LITERAL"
>OVER</TT
> clauses, but
they all act on the same collection of rows defined by this virtual table.
</P
><P
> We already saw that <TT
CLASS="LITERAL"
>ORDER BY</TT
> can be omitted if the ordering
of rows is not important. It is also possible to omit <TT
CLASS="LITERAL"
>PARTITION
BY</TT
>, in which case there is just one partition containing all the 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
<I
CLASS="FIRSTTERM"
>window frame</I
>. Many (but not all) window functions act only
on the rows of the window frame, rather than of the whole partition.
By default, if <TT
CLASS="LITERAL"
>ORDER BY</TT
> 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
<TT
CLASS="LITERAL"
>ORDER BY</TT
> clause. When <TT
CLASS="LITERAL"
>ORDER BY</TT
> is omitted the
default frame consists of all rows in the partition.
<A
NAME="AEN1050"
HREF="#FTN.AEN1050"
><SPAN
CLASS="footnote"
>[1]</SPAN
></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 <TT
CLASS="LITERAL"
>ORDER BY</TT
> in the <TT
CLASS="LITERAL"
>OVER</TT
>
clause, the window frame is the same as the partition, which for lack of
<TT
CLASS="LITERAL"
>PARTITION BY</TT
> 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 <TT
CLASS="LITERAL"
>ORDER BY</TT
> 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 <TT
CLASS="LITERAL"
>SELECT</TT
> list
and the <TT
CLASS="LITERAL"
>ORDER BY</TT
> clause of the query. They are forbidden
elsewhere, such as in <TT
CLASS="LITERAL"
>GROUP BY</TT
>, <TT
CLASS="LITERAL"
>HAVING</TT
>
and <TT
CLASS="LITERAL"
>WHERE</TT
> clauses. This is because they logically
execute after the processing of those clauses. Also, window functions
execute after regular 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
<TT
CLASS="LITERAL"
>rank</TT
> less than 3.
</P
><P
> When a query involves multiple window functions, it is possible to write
out each one with a separate <TT
CLASS="LITERAL"
>OVER</TT
> 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 <TT
CLASS="LITERAL"
>WINDOW</TT
> clause and then referenced in <TT
CLASS="LITERAL"
>OVER</TT
>.
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
HREF="sql-expressions.html#SYNTAX-WINDOW-FUNCTIONS"
>Section 4.2.8</A
>,
<A
HREF="functions-window.html"
>Section 9.21</A
>,
<A
HREF="queries-table-expressions.html#QUERIES-WINDOW"
>Section 7.2.4</A
>, and the
<A
HREF="sql-select.html"
>SELECT</A
> reference page.
</P
></DIV
><H3
CLASS="FOOTNOTES"
>Notes</H3
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><A
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HREF="tutorial-window.html#AEN1050"
><SPAN
CLASS="footnote"
>[1]</SPAN
></A
></TD
><TD
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><P
> There are options to define the window frame in other ways, but
this tutorial does not cover them. See
<A
HREF="sql-expressions.html#SYNTAX-WINDOW-FUNCTIONS"
>Section 4.2.8</A
> for details.
</P
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