Windowing

A major feature of the ANSI SQL extensions for OLAP is a construct called a window. This windowing extension let users divide result sets of a query (or a logical partition of a query) into groups of rows called partitions and determine subsets of rows to aggregate with respect to the current row.

You can use three classes of window functions with a window: ranking functions, the row numbering function, and window aggregate functions.

<WINDOWED TABLE FUNCTION TYPE> ::=
  <RANK FUNCTION TYPE> <LEFT PAREN> <RIGHT PAREN>
  | ROW_NUMBER <LEFT PAREN> <RIGHT PAREN>
  | <WINDOW AGGREGATE FUNCTION>

See also “Grammar rule 6”.

Windowing extensions specify a window function type over a window name or specification and are applied to partitioned result sets within the scope of a single query expression. A window partition is a subset of rows returned by a query, as defined by one or more columns in a special OVER clause:

olap_function() OVER (PARTITION BY col1, col2...)

Windowing operations let you establish information such as the ranking of each row within its partition, the distribution of values in rows within a partition, and similar operations. Windowing also lets you compute moving averages and sums on your data, enhancing the ability to evaluate your data and its impact on your operations.

An OLAP window’s three essential parts

The OLAP windows comprise three essential aspects: window partitioning, window ordering, and window framing. Each has a significant impact on the specific rows of data visible in a window at any point in time. Meanwhile, the OLAP OVER clause differentiates OLAP functions from other analytic or reporting functions with three distinct capabilities:

Defining window partitions (PARTITION BY clause). See “Window partitioning”.
Ordering rows within partitions (ORDER BY clause). See “Window ordering”.
Defining window frames (ROWS/RANGE specification). See “Window framing”.

To specify multiple windows functions, and to avoid redundant window definitions, you can specify a name for an OLAP window specifications. In this usage, the keyword, WINDOW, is followed by at least one window definition, separated by commas. A window definition includes the name by which the window is known in the query and the details from the windows specification, which lets you to define window partitioning, ordering, and framing:

<WINDOW CLAUSE> ::= <WINDOW DEFINITION LIST>

<WINDOW DEFINITION LIST> ::=
  <WINDOW DEFINITION> [ { <COMMA> <WINDOW DEFINITION>
  } . . . ]

<WINDOW DEFINITION> ::=
  <NEW WINDOW NAME> AS <WINDOW SPECIFICATION>

<WINDOW SPECIFICATION DETAILS> ::=
  [ <EXISTING WINDOW NAME> ]
  [ <WINDOW PARTITION CLAUSE> ]
  [ <WINDOW ORDER CLAUSE> ]
  [ <WINDOW FRAME CLAUSE> ]

For each row in a window partition, users can define a window frame, which may vary the specific range of rows used to perform any computation on the current row of the partition. The current row provides the reference point for determining the start and end points of the window frame.

Window specifications can be based on either a physical number of rows using a window specification that defines a window frame unit of ROWS or a logical interval of a numeric value, using a window specification that defines a window frame unit of RANGE. See “Window framing”.

Within OLAP windowing operations, you can use the following functional categories: