PostgreSQL 9.4.1 Documentation | |||
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*Window functions* provide the ability
to perform calculations across sets of rows that are related to
the current query row. See Section
3.5 for an introduction to this feature, and Section 4.2.8
for syntax details.

The built-in window functions are listed in Table 9-53.
Note that these functions must be invoked using window
function syntax; that is an `OVER` clause
is required.

In addition to these functions, any built-in or user-defined
normal aggregate function (but not ordered-set or
hypothetical-set aggregates) can be used as a window function;
see Section 9.20 for a
list of the built-in aggregates. Aggregate functions act as
window functions only when an `OVER`
clause follows the call; otherwise they act as regular
aggregates.

Table 9-53. General-Purpose Window Functions

Function | Return Type | Description |
---|---|---|

`row_number()` |
bigint |
number of the current row within its partition, counting from 1 |

`rank()` |
bigint |
rank of the current row with gaps; same as
`row_number` of its first
peer |

`dense_rank()` |
bigint |
rank of the current row without gaps; this function counts peer groups |

`percent_rank()` |
double precision |
relative rank of the current row: (`rank` - 1) / (total rows - 1) |

`cume_dist()` |
double precision |
relative rank of the current row: (number of rows preceding or peer with current row) / (total rows) |

`ntile(` |
integer |
integer ranging from 1 to the argument value, dividing the partition as equally as possible |

`lag(` |
same type as value |
returns value
evaluated at the row that is offset rows before the current row
within the partition; if there is no such row, instead
return default. Both
offset and default are evaluated with respect
to the current row. If omitted, offset defaults to 1 and default to null |

`lead(` |
same type as value |
returns value
evaluated at the row that is offset rows after the current row
within the partition; if there is no such row, instead
return default. Both
offset and default are evaluated with respect
to the current row. If omitted, offset defaults to 1 and default to null |

`first_value(` |
same type as value |
returns value
evaluated at the row that is the first row of the window
frame |

`last_value(` |
same type as value |
returns value
evaluated at the row that is the last row of the window
frame |

`nth_value(` |
same type as value |
returns value
evaluated at the row that is the nth row of the window frame
(counting from 1); null if no such row |

All of the functions listed in Table 9-53
depend on the sort ordering specified by the `ORDER BY` clause of the associated window
definition. Rows that are not distinct in the `ORDER BY` ordering are said to be *peers*; the four ranking functions are defined so
that they give the same answer for any two peer rows.

Note that `first_value`

,
`last_value`

, and `nth_value`

consider 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_value`

and
sometimes also `nth_value`

. You can
redefine the frame by adding a suitable frame specification
(`RANGE` or `ROWS`)
to the `OVER` clause. See Section 4.2.8
for 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 BY` and 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 BY` or use
`ROWS BETWEEN UNBOUNDED PRECEDING AND
UNBOUNDED FOLLOWING`. Other frame specifications can be used
to obtain other effects.

Note:The SQL standard defines aRESPECT NULLSorIGNORE NULLSoption for`lead`

,`lag`

,`first_value`

,`last_value`

, and`nth_value`

. This is not implemented in PostgreSQL: the behavior is always the same as the standard's default, namelyRESPECT NULLS. Likewise, the standard'sFROM FIRSTorFROM LASToption for`nth_value`

is not implemented: only the defaultFROM FIRSTbehavior is supported. (You can achieve the result ofFROM LASTby reversing theORDER BYordering.)