MySQL :: MySQL 8.4 Reference Manual :: 14.20.4 Named Windows (original) (raw)
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 aWINDOW
clause. If present in a query, theWINDOW
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,windowname
is the window name, and_windowspec
_ is the same type of window specification as given between the parentheses of anOVER
clause, as described inSection 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 theOVER
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 usingWINDOW
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 theWINDOW
clause, rather than multipleOVER
clause definitions.
If an OVER
clause uses OVER (_`windowname`_ ...)
rather than OVER_`windowname`_
, 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 theOVER
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, theOVER
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 specifiesPARTITION BY
for a named window that already hasPARTITION BY
:
OVER (w PARTITION BY year)
... WINDOW w AS (PARTITION BY country)
The definition of a named window can itself begin with a_windowname
_. 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)