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Window function (SQL)In SQL, a window function or analytic function[1] is a function which uses values from one or multiple rows to return a value for each row. (This contrasts with an aggregate function, which returns a single value for multiple rows.) Window functions have an OVER clause; any function without an OVER clause is not a window function, but rather an aggregate or single-row (scalar) function.[2] ExampleAs an example, here is a query which uses a window function to compare the salary of each employee with the average salary of their department (example from the PostgreSQL documentation):[3] SELECT depname, empno, salary, avg(salary) OVER (PARTITION BY depname) FROM empsalary;
Output: 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) The Window functions are evaluated after aggregation (after the SyntaxAccording to the PostgreSQL documentation, a window function has the syntax of one of the following:[4] function_name ([expression [, expression ... ]]) OVER window_name
function_name ([expression [, expression ... ]]) OVER ( window_definition )
function_name ( * ) OVER window_name
function_name ( * ) OVER ( window_definition )
where [ existing_window_name ]
[ PARTITION BY expression [, ...] ]
[ ORDER BY expression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...] ]
[ frame_clause ]
{ RANGE | ROWS | GROUPS } frame_start [ frame_exclusion ]
{ RANGE | ROWS | GROUPS } BETWEEN frame_start AND frame_end [ frame_exclusion ]
Notation:
ExampleWindow functions allow access to data in the records right before and after the current record.[5][6][7][8] A window function defines a frame or window of rows with a given length around the current row, and performs a calculation across the set of data in the window.[9][10] NAME |
------------
Aaron| <-- Preceding (unbounded)
Andrew|
Amelia|
James|
Jill|
Johnny| <-- 1st preceding row
Michael| <-- Current row
Nick| <-- 1st following row
Ophelia|
Zach| <-- Following (unbounded)
In the above table, the next query extracts for each row the values of a window with one preceding and one following row: SELECT
LAG(name, 1)
OVER(ORDER BY name) "prev",
name,
LEAD(name, 1)
OVER(ORDER BY name) "next"
FROM people
ORDER BY name
The result query contains the following values: | PREV | NAME | NEXT | |----------|----------|----------| | (null)| Aaron| Andrew| | Aaron| Andrew| Amelia| | Andrew| Amelia| James| | Amelia| James| Jill| | James| Jill| Johnny| | Jill| Johnny| Michael| | Johnny| Michael| Nick| | Michael| Nick| Ophelia| | Nick| Ophelia| Zach| | Ophelia| Zach| (null)| HistoryWindow functions were incorporated into the SQL:2003 standard and had functionality expanded in later specifications.[11] Support for particular database implementations was added as follows:
See alsoReferences
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