_Window functions_provide the ability to perform calculations across sets of rows that are related to the current query row. SeeSection 3.5for an introduction to this feature, andSection 4.2.8for syntax details.
The built-in window functions are listed inTable 9.57. Note that these functions_must_be invoked using window function syntax, i.e., an
OVERclause is required.
In addition to these functions, any built-in or user-defined general-purpose or statistical aggregate (i.e., not ordered-set or hypothetical-set aggregates) can be used as a window function; seeSection 9.20for a list of the built-in aggregates. Aggregate functions act as window functions only when an
OVERclause follows the call; otherwise they act as non-window aggregates and return a single row for the entire set.
Table 9.57. General-Purpose Window Functions
number of the current row within its partition, counting from 1
rank of the current row with gaps; same as
rank of the current row without gaps; this function counts peer groups
relative rank of the current row: (
cumulative distribution: (number of partition rows preceding or peer with current row) / total partition rows
integer ranging from 1 to the argument value, dividing the partition as equally as possible
All of the functions listed inTable 9.57depend on the sort ordering specified by the
ORDER BYclause of the associated window definition. Rows that are not distinct when considering only the
ORDER BYcolumns are said to bepeers. The four ranking functions (including
cume_dist) are defined so that they give the same answer for all peer rows.
nth_valueconsider only the rows within the“window frame”, which by default contains the rows from the start of the partition through the last peer of the current row. This is likely to give unhelpful results for
last_valueand sometimes also
nth_value. You can redefine the frame by adding a suitable frame specification (
ROWS) to the
OVERclause. SeeSection 4.2.8for more information about frame specifications.
When an aggregate function is used as a window function, it aggregates over the rows within the current row's window frame. An aggregate used with
ORDER BYand the default window frame definition produces a“running sum”type of behavior, which may or may not be what's wanted. To obtain aggregation over the whole partition, omit
ORDER BYor use
ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING. Other frame specifications can be used to obtain other effects.
The SQL standard defines a
IGNORE NULLSoption for
nth_value. This is not implemented inPostgreSQL: the behavior is always the same as the standard's default, namely
RESPECT NULLS. Likewise, the standard's
FROM LASToption for
nth_valueis not implemented: only the default
FROM FIRSTbehavior is supported. (You can achieve the result of
FROM LASTby reversing the
cume_distcomputes the fraction of partition rows that are less than or equal to the current row and its peers, while
percent_rankcomputes the fraction of partition rows that are less than the current row, assuming the current row does not exist in the partition.