14.20.4 Named Windows

Windows can be defined and given names by which to refer to them in OVER clauses. To do this, use a WINDOW clause. If present in a query, the WINDOW clause falls between the positions of the HAVING and ORDER BY clauses, and has this syntax:

WINDOW window_name AS (window_spec)
    [, window_name AS (window_spec)] ...

For each window definition, window_name is the window name, and window_spec is the same type of window specification as given between the parentheses of an OVER clause, as described in Section 14.20.2, “Window Function Concepts and Syntax”:

window_spec:
    [window_name] [partition_clause] [order_clause] [frame_clause]

A WINDOW clause is useful for queries in which multiple OVER clauses would otherwise define the same window. Instead, you can define the window once, give it a name, and refer to the name in the OVER clauses. Consider this query, which defines the same window multiple times:

SELECT
  val,
  ROW_NUMBER() OVER (ORDER BY val) AS 'row_number',
  RANK()       OVER (ORDER BY val) AS 'rank',
  DENSE_RANK() OVER (ORDER BY val) AS 'dense_rank'
FROM numbers;

The query can be written more simply by using WINDOW to define the window once and referring to the window by name in the OVER clauses:

SELECT
  val,
  ROW_NUMBER() OVER w AS 'row_number',
  RANK()       OVER w AS 'rank',
  DENSE_RANK() OVER w AS 'dense_rank'
FROM numbers
WINDOW w AS (ORDER BY val);

A named window also makes it easier to experiment with the window definition to see the effect on query results. You need only modify the window definition in the WINDOW clause, rather than multiple OVER clause definitions.

If an OVER clause uses OVER (window_name ...) rather than OVER window_name, the named window can be modified by the addition of other clauses. For example, this query defines a window that includes partitioning, and uses ORDER BY in the OVER clauses to modify the window in different ways:

SELECT
  DISTINCT year, country,
  FIRST_VALUE(year) OVER (w ORDER BY year ASC) AS first,
  FIRST_VALUE(year) OVER (w ORDER BY year DESC) AS last
FROM sales
WINDOW w AS (PARTITION BY country);

An OVER clause can only add properties to a named window, not modify them. If the named window definition includes a partitioning, ordering, or framing property, the OVER clause that refers to the window name cannot also include the same kind of property or an error occurs:

This construct is permitted because the window definition and the referring OVER clause do not contain the same kind of properties:
```
OVER (w ORDER BY country)
... WINDOW w AS (PARTITION BY country)
```
This construct is not permitted because the OVER clause specifies PARTITION BY for a named window that already has PARTITION BY:
```
OVER (w PARTITION BY year)
... WINDOW w AS (PARTITION BY country)
```

The definition of a named window can itself begin with a window_name. In such cases, forward and backward references are permitted, but not cycles:

This is permitted; it contains forward and backward references but no cycles:
```
WINDOW w1 AS (w2), w2 AS (), w3 AS (w1)
```
This is not permitted because it contains a cycle:
```
WINDOW w1 AS (w2), w2 AS (w3), w3 AS (w1)
```