Window functions

By using window functions, you can create analytic business queries more efficiently. Window functions operate on a partition or "window" of a result set, and return a value for every row in that window. In contrast, non-windowed functions perform their calculations with respect to every row in the result set. Unlike group functions that aggregate result rows, window functions retain all rows in the table expression.

The values returned are calculated by using values from the sets of rows in that window. For each row in the table, the window defines a set of rows that is used to compute additional attributes. A window is defined using a window specification (the OVER clause), and is based on three main concepts:

• Window partitioning, which forms groups of rows (PARTITION clause)

• Window ordering, which defines an order or sequence of rows within each partition (ORDER BY clause)

• Window frames, which are defined relative to each row to further restrict the set of rows (ROWS specification)

Window functions are the last set of operations performed in a query except for the final ORDER BY clause. All joins and all WHERE, GROUP BY, and HAVING clauses are completed before the window functions are processed. Therefore, window functions can appear only in the select list or ORDER BY clause. You can use multiple window functions within a single query with different frame clauses. You can also use window functions in other scalar expressions, such as CASE.

Window function syntax summary

Window functions follow a standard syntax, which is as follows.

function (expression) OVER (
[ PARTITION BY expr_list ]
[ ORDER BY order_list [ frame_clause ] ] )

Here, function is one of the functions described in this section.

The expr_list is as follows.

expression | column_name [, expr_list ]

The order_list is as follows.

expression | column_name [ ASC | DESC ] 
[ NULLS FIRST | NULLS LAST ]
[, order_list ]

The frame_clause is as follows.

ROWS
{ UNBOUNDED PRECEDING | unsigned_value PRECEDING | CURRENT ROW } |

{ BETWEEN
{ UNBOUNDED PRECEDING | unsigned_value { PRECEDING | FOLLOWING } | CURRENT ROW}
AND
{ UNBOUNDED FOLLOWING | unsigned_value { PRECEDING | FOLLOWING } | CURRENT ROW }}

Arguments

function

The window function. For details, see the individual function descriptions.

OVER

The clause that defines the window specification. The OVER clause is mandatory for window functions, and differentiates window functions from other SQL functions.

PARTITION BY expr_list

(Optional) The PARTITION BY clause subdivides the result set into partitions, much like the GROUP BY clause. If a partition clause is present, the function is calculated for the rows in each partition. If no partition clause is specified, a single partition contains the entire table, and the function is computed for that complete table.

The ranking functions DENSE_RANK, NTILE, RANK, and ROW_NUMBER require a global comparison of all the rows in the result set. When a PARTITION BY clause is used, the query optimizer can run each aggregation in parallel by spreading the workload across multiple slices according to the partitions. If the PARTITION BY clause is not present, the aggregation step must be run serially on a single slice, which can have a significant negative impact on performance, especially for large clusters.

Amazon Redshift doesn't support string literals in PARTITION BY clauses.

ORDER BY order_list

(Optional) The window function is applied to the rows within each partition sorted according to the order specification in ORDER BY. This ORDER BY clause is distinct from and completely unrelated to ORDER BY clauses in the frame_clause. The ORDER BY clause can be used without the PARTITION BY clause.

For ranking functions, the ORDER BY clause identifies the measures for the ranking values. For aggregation functions, the partitioned rows must be ordered before the aggregate function is computed for each frame. For more about window function types, see Window functions.

Column identifiers or expressions that evaluate to column identifiers are required in the order list. Neither constants nor constant expressions can be used as substitutes for column names.

NULLS values are treated as their own group, sorted and ranked according to the NULLS FIRST or NULLS LAST option. By default, NULL values are sorted and ranked last in ASC ordering, and sorted and ranked first in DESC ordering.

Amazon Redshift doesn't support string literals in ORDER BY clauses.

If the ORDER BY clause is omitted, the order of the rows is nondeterministic.

Note

In any parallel system such as Amazon Redshift, when an ORDER BY clause doesn't produce a unique and total ordering of the data, the order of the rows is nondeterministic. That is, if the ORDER BY expression produces duplicate values (a partial ordering), the return order of those rows might vary from one run of Amazon Redshift to the next. In turn, window functions might return unexpected or inconsistent results. For more information, see Unique ordering of data for window functions.

column_name

Name of a column to be partitioned by or ordered by.

ASC | DESC

Option that defines the sort order for the expression, as follows:

• ASC: ascending (for example, low to high for numeric values and 'A' to 'Z' for character strings). If no option is specified, data is sorted in ascending order by default.

• DESC: descending (high to low for numeric values; 'Z' to 'A' for strings).

NULLS FIRST | NULLS LAST

Option that specifies whether NULLS should be ordered first, before non-null values, or last, after non-null values. By default, NULLS are sorted and ranked last in ASC ordering, and sorted and ranked first in DESC ordering.

frame_clause

For aggregate functions, the frame clause further refines the set of rows in a function's window when using ORDER BY. It enables you to include or exclude sets of rows within the ordered result. The frame clause consists of the ROWS keyword and associated specifiers.

The frame clause doesn't apply to ranking functions. Also, the frame clause isn't required when no ORDER BY clause is used in the OVER clause for an aggregate function. If an ORDER BY clause is used for an aggregate function, an explicit frame clause is required.

When no ORDER BY clause is specified, the implied frame is unbounded, equivalent to ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING.

ROWS

This clause defines the window frame by specifying a physical offset from the current row.

This clause specifies the rows in the current window or partition that the value in the current row is to be combined with. It uses arguments that specify row position, which can be before or after the current row. The reference point for all window frames is the current row. Each row becomes the current row in turn as the window frame slides forward in the partition.

The frame can be a simple set of rows up to and including the current row.

{UNBOUNDED PRECEDING | offset PRECEDING | CURRENT ROW}

Or it can be a set of rows between two boundaries.

BETWEEN
{ UNBOUNDED PRECEDING | offset { PRECEDING | FOLLOWING } | CURRENT ROW }
AND
{ UNBOUNDED FOLLOWING | offset { PRECEDING | FOLLOWING } | CURRENT ROW }

UNBOUNDED PRECEDING indicates that the window starts at the first row of the partition; offset PRECEDING indicates that the window starts a number of rows equivalent to the value of offset before the current row. UNBOUNDED PRECEDING is the default.

CURRENT ROW indicates the window begins or ends at the current row.

UNBOUNDED FOLLOWING indicates that the window ends at the last row of the partition; offset FOLLOWING indicates that the window ends a number of rows equivalent to the value of offset after the current row.

offset identifies a physical number of rows before or after the current row. In this case, offset must be a constant that evaluates to a positive numeric value. For example, 5 FOLLOWING ends the frame five rows after the current row.

Where BETWEEN is not specified, the frame is implicitly bounded by the current row. For example, ROWS 5 PRECEDING is equal to ROWS BETWEEN 5 PRECEDING AND CURRENT ROW. Also, ROWS UNBOUNDED FOLLOWING is equal to ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING.

Note

You can't specify a frame in which the starting boundary is greater than the ending boundary. For example, you can't specify any of the following frames.

between 5 following and 5 preceding
between current row and 2 preceding
between 3 following and current row

Unique ordering of data for window functions

If an ORDER BY clause for a window function doesn't produce a unique and total ordering of the data, the order of the rows is nondeterministic. If the ORDER BY expression produces duplicate values (a partial ordering), the return order of those rows can vary in multiple runs. In this case, window functions can also return unexpected or inconsistent results.

For example, the following query returns different results over multiple runs. These different results occur because order by dateid doesn't produce a unique ordering of the data for the SUM window function.

select dateid, pricepaid,
sum(pricepaid) over(order by dateid rows unbounded preceding) as sumpaid
from sales
group by dateid, pricepaid;

dateid | pricepaid |   sumpaid
--------+-----------+-------------
1827 |   1730.00 |     1730.00
1827 |    708.00 |     2438.00
1827 |    234.00 |     2672.00
...

select dateid, pricepaid,
sum(pricepaid) over(order by dateid rows unbounded preceding) as sumpaid
from sales
group by dateid, pricepaid;

dateid | pricepaid |   sumpaid
--------+-----------+-------------
1827 |    234.00 |      234.00
1827 |    472.00 |      706.00
1827 |    347.00 |     1053.00
...

In this case, adding a second ORDER BY column to the window function can solve the problem.

select dateid, pricepaid,
sum(pricepaid) over(order by dateid, pricepaid rows unbounded preceding) as sumpaid
from sales
group by dateid, pricepaid;

dateid | pricepaid | sumpaid
--------+-----------+---------
1827 |    234.00 |  234.00
1827 |    337.00 |  571.00
1827 |    347.00 |  918.00
...

Supported functions

Amazon Redshift supports two types of window functions: aggregate and ranking.

Following are the supported aggregate functions:

• AVG window function

• COUNT window function

• CUME_DIST window function

• DENSE_RANK window function

• FIRST_VALUE and LAST_VALUE window functions

• LAG window function

• LEAD window function

• LISTAGG window function

• MAX window function

• MEDIAN window function

• MIN window function

• NTH_VALUE window function

• PERCENTILE_CONT window function

• PERCENTILE_DISC window function

• RATIO_TO_REPORT window function

• STDDEV_SAMP and STDDEV_POP window functions (STDDEV_SAMP and STDDEV are synonyms)

• SUM window function

• VAR_SAMP and VAR_POP window functions (VAR_SAMP and VARIANCE are synonyms)

Following are the supported ranking functions:

• DENSE_RANK window function

• NTILE window function

• PERCENT_RANK window function

• RANK window function

• ROW_NUMBER window function

Sample table for window function examples

You can find specific window function examples with each function description. Some of the examples use a table named WINSALES, which contains 11 rows, as shown following.

SALESID	DATEID	SELLERID	BUYERID	QTY	QTY_SHIPPED
30001	8/2/2003	3	B	10	10
10001	12/24/2003	1	C	10	10
10005	12/24/2003	1	A	30
40001	1/9/2004	4	A	40
10006	1/18/2004	1	C	10
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
30004	4/18/2004	3	B	20
30007	9/7/2004	3	C	30

The following script creates and populates the sample WINSALES table.

CREATE TABLE winsales(
  salesid int,
  dateid date,
  sellerid int,
  buyerid char(10),
  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);