Window Functions

The CData Virtuality Server supports ANSI SQL 2003 window functions. A window function allows an aggregate function to be applied to a subset of the result set without the need for a GROUP BY clause. A window function is similar to an aggregate function, but requires the use of an OVER clause or window specification.

Usage

aggregate|ranking OVER ([PARTITION BY ...] [ORDER BY ...] [<FRAME-CLAUSE>])

aggregate can be any aggregate function. Ranking can be one of ROW_NUMBER() , RANK(), DENSE_RANK().

Syntax Rules

• Window functions can only appear in the SELECT and ORDER BY clauses of a query expression;
• Window functions cannot be nested in one another;
• Partitioning and order by expressions cannot contain subqueries or outer references;
• The ranking (ROW_NUMBER, RANK, DENSE_RANK) functions require the use of the window specification ORDER BY clause;
• An XMLAGG ORDER BY clause cannot be used when windowed;
• The window specification ORDER BY clause cannot reference alias names or use positional ordering;
• Windowed aggregates may not use DISTINCT if the window specification is ordered;
• The Frame Clause can only be used in combination with the Partition Clause (at least one of PARTITION BY or ORDER BY should be present).

Frame Clause

The frame clause allows the user to specify a dynamic group of rows, or a range, inside the partition (analogous to a sliding frame in a window). It can be used in two ways:

• [ RANGE | ROWS ] frame_start

• [ RANGE | ROWS ] BETWEEN frame_start AND frame_end

Where frame_start and frame_end can be one of:

• UNBOUNDED PRECEDING

• value PRECEDING (ROWS only)

• CURRENT ROW

• value FOLLOWING (ROWS only)

• UNBOUNDED FOLLOWING

Restrictions are that frame_start cannot be UNBOUNDED FOLLOWING, frame_end cannot be UNBOUNDED PRECEDING, and the frame_end choice cannot appear earlier in the above list than the frame_start choice. For example, RANGE BETWEEN CURRENT ROW AND value PRECEDING is not allowed.

Examples of Usage

Let us assume we have the following table called my_table:

SELECT x, SUM(y) OVER (PARTITION BY y ROWS BETWEEN 2 PRECEDING AND CURRENT ROW) AS window_column 
FROM data_source.my_table;
 
-- Result:
-- |-----------|---------------|
-- |     x     | window_column |
-- |-----------|---------------|
-- |     1     |       1       |
-- |     2     |       2       |
-- |     3     |       3       |
-- |     4     |       2       |
-- |     5     |       3       |
-- |-----------|---------------|
 
 
SELECT x, COUNT(y) OVER (PARTITION BY y RANGE BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING) AS window_column 
FROM data_source.my_table;
 
-- Result:
-- |-----------|---------------|
-- |     x     | window_column |
-- |-----------|---------------|
-- |     1     |       3       |
-- |     2     |       3       |
-- |     3     |       3       |
-- |     4     |       1       |
-- |     5     |       1       |
-- |-----------|---------------|
 
 
SELECT x, y*100/SUM(y) OVER (PARTITION BY y RANGE BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING) AS window_column 
FROM data_source.my_table;
 
-- Result:
-- |-----------|---------------|
-- |     x     | window_column |
-- |-----------|---------------|
-- |     1     |       33      |
-- |     2     |       33      |
-- |     3     |       33      |
-- |     4     |       100     |
-- |     5     |       100     |
-- |-----------|---------------|

Function Definitions

Processing

Window functions are logically processed just before creating the output from the SELECT clause. Window functions can use nested aggregates if a GROUP BY clause is present. The is no guaranteed effect on the output ordering from the presence of window functions. The SELECT statement must have an ORDER BY clause to have a predictable ordering.

The CData Virtuality Server will process all window functions with the same window specification together. Generally, a full pass over the row values coming into the SELECT clause will be required for each unique window specification. For each window specification, the values will be grouped according to the PARTITION BY clause. If no PARTITION BY clause is specified, the entire input is treated as a single partition. The output value is determined based upon the current row value, its peers (that is, rows that are the same with respect to their ordering), and all prior row values based upon ordering in the partition. The ROW_NUMBER function will assign a unique value to every row regardless of the number of peers.

SELECT name, salary, max(salary) over (partition by name) as max_sal,
          rank() over (order by salary) as rank, dense_rank() over (order by salary) as dense_rank,
          row_number() over (order by salary) as row_num 
FROM data_source.employees

See Also

Duplicate Removal with ROW_NUMBER() and PARTITION to see how window functions can be used for removing duplicates