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Overview

All window functions utilise a set of clauses specific for them, some of which are mandatory while others are optional.

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OVER Clause

When it comes to required ones, there is the OVER clause, which defines a window or user-specified set of rows within a query result set. It is a mandatory element of window functions, defining the window specification and differentiating them from other SQL functions.

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Syntax

The syntax for this clause looks as follows: 
OVER (PARTITION BY rows1 ORDER BY rows2)
where, the PARTITION BY clause is a list of expressions interpreted in much the same fashion as the elements of a GROUP BY clause, with major exception that they are always simple expressions and never the name or number of an output column. Another difference is that these expressions can contain aggregate function calls, which are not allowed in a regular GROUP BY clause (they are allowed here because windowing occurs after grouping and aggregation) [ PARTITION BY expression [, ...] ] (optional window partition) The ORDER BY clause used in the OVER clause above is a list of expressions interpreted in much the same fashion as the elements of a statement-level ORDER BY clause, except that the expressions are always taken as simple expressions and never the name or number of an output column. [ ORDER BY expression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...] ] (optional window ordering)

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WINDOW Clause

In terms of window functions’ optional clauses, there is the WINDOW clause that defines one or more named window specification, as a window_name and window_definition pair.

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Syntax

The syntax for this clause looks as follows: 
WINDOW window_name AS (window_definition) [, ...]
where window_name is a name that can be referenced from the OVER clauses or subsequent window definition. There are a few important things to keep in mind here:
  • The window_definition may use an existing_window_name to refer to a previous window_definition in the WINDOW clause, but the previous window_definition must not specify a frame clause
  • The window_definition copies the PARTITION BY clause and ORDER BY clause from previous window_definition, but it cannot specify its own PARTION BY clause, and can specify an ORDER BY clause if the previous window_definition does not have one.
[ existing_window_name ] [ PARTITION BY clause ] [ ORDER BY clause ] [ frame clause ] (all arguments are optional)
The window_definition without arguments defines a window with all rows without partition and ordering
The frame clause referenced above defines the window frame for window functions that depend on the frame (not all do). The window frame is a set of related rows for each row of the query (called the current row).
  • { RANGE | ROWS | GROUPS } frame_start [ frame_exclusion ]
  • { RANGE | ROWS | GROUPS } BETWEEN frame_start AND frame_end [ frame_exclusion ]
The frame clause of the window specification is limited to the ROWS clause without frame exclusion one
There are a couple of things, to keep in mind here:
  • frame_start and frame_end can be one of: UNBOUNDED PRECEDING, offset PRECEDING, CURRENT ROW, offset FOLLOWING, UNBOUNDED FOLLOWING.
  • If frame_end is omitted it defaults to CURRENT ROW. Restrictions here are as follows:
    • frame_start cannot be UNBOUNDED FOLLOWING
    • frame_end cannot be UNBOUNDED PRECEDING
    • frame_end choice cannot appear earlier in the above list of frame_start and frame_end options than the frame_start choice does
In ROWS mode, CURRENT ROW means that the frame starts or ends with the current row, the offset is an integer indicating that the frame starts or ends that many rows before or after the current row.
Beware that the ROWS mode can produce unpredictable results if the ORDER BY ordering does not order the rows uniquely

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Examples

For the needs of this section, we will create the winsales table that stores details about some sales transactions: 
CREATE TABLE winsales(
    salesid int,
    dateid date,
    sellerid int,
    buyerid text,
    qty int,
    qty_shipped int);
INSERT INTO winsales VALUES
    (30001, '8/2/2003', 3, 'b', 10, 10),
    (10001, '12/24/2003', 1, 'c', 10, 10),
    (10005, '12/24/2003', 1, 'a', 30, null),
    (40001, '1/9/2004', 4, 'a', 40, null),
    (10006, '1/18/2004', 1, 'c', 10, null),
    (20001, '2/12/2004', 2, 'b', 20, 20),
    (40005, '2/12/2004', 4, 'a', 10, 10),
    (20002, '2/16/2004', 2, 'c', 20, 20),
    (30003, '4/18/2004', 3, 'b', 15, null),
    (30004, '4/18/2004', 3, 'b', 20, null),
    (30007, '9/7/2004', 3, 'c', 30, null);

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OVER Clause in Window Functions with Window Definition, PARTITION BY and ORDER BY clauses

In this example, we will focus on executing a window function with the OVER clause, window definition and PARTITION BY and ORDER BY clauses: 
SELECT *
  SUM(qty) OVER (PARTITION BY sellerid) AS seller_qty 
FROM winsales 
ORDER BY sellerid, salesid;
Here’s the output for the above code: 
  salesid |   dateid   | sellerid | buyerid | qty | qty_shipped | seller_qty 
---------+------------+----------+---------+-----+-------------+------------
   10001 | 2003-12-24 |        1 | c       |  10 |          10 |         50
   10005 | 2003-12-24 |        1 | a       |  30 |             |         50
   10006 | 2004-01-18 |        1 | c       |  10 |             |         50
   20001 | 2004-02-12 |        2 | b       |  20 |          20 |         40
   20002 | 2004-02-16 |        2 | c       |  20 |          20 |         40
   30001 | 2003-08-02 |        3 | b       |  10 |          10 |         75
   30003 | 2004-04-18 |        3 | b       |  15 |             |         75
   30004 | 2004-04-18 |        3 | b       |  20 |             |         75
   30007 | 2004-09-07 |        3 | c       |  30 |             |         75
   40001 | 2004-01-09 |        4 | a       |  40 |             |         50
   40005 | 2004-02-12 |        4 | a       |  10 |          10 |         50
(11 rows)

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OVER Clause in Window Functions with Window Name, PARTITION BY and ORDER BY clauses

In this example, we will focus on executing a window function with the OVER clause, window name and PARITION BY and ORDER BY clauses: 
SELECT *
  SUM(qty) OVER seller AS seller_qty 
FROM winsales WINDOW seller AS (PARTITION BY sellerid) 
ORDER BY sellerid, salesid;
When executing the code above, we will get the following output: 
  salesid |   dateid   | sellerid | buyerid | qty | qty_shipped | seller_qty 
---------+------------+----------+---------+-----+-------------+------------
   10001 | 2003-12-24 |        1 | c       |  10 |          10 |         50
   10005 | 2003-12-24 |        1 | a       |  30 |             |         50
   10006 | 2004-01-18 |        1 | c       |  10 |             |         50
   20001 | 2004-02-12 |        2 | b       |  20 |          20 |         40
   20002 | 2004-02-16 |        2 | c       |  20 |          20 |         40
   30001 | 2003-08-02 |        3 | b       |  10 |          10 |         75
   30003 | 2004-04-18 |        3 | b       |  15 |             |         75
   30004 | 2004-04-18 |        3 | b       |  20 |             |         75
   30007 | 2004-09-07 |        3 | c       |  30 |             |         75
   40001 | 2004-01-09 |        4 | a       |  40 |             |         50
   40005 | 2004-02-12 |        4 | a       |  10 |          10 |         50
(11 rows)
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