Window aggregate functions
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Window aggregate functions allow aggregate calculations over a "window" of rows defined by the OVER clause. This allows aggregates to be calculated for each row without grouping, providing more detail. The document provides two examples: 1) Using window functions to calculate a grand total and customer total for each order. 2) Using a window frame to calculate a running total that accumulates values within each customer partition up to the current row. Window functions add flexibility over regular aggregates by allowing per-row calculations over a window of rows.
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Window aggregate functions
- 1. + Window Aggregate Functions Dr. Silvio Sangineto Business Analyst E-Mail: [email protected] Web-Site: www.silviosangineto.it
- 2. + Introduction Window aggregate functions are the same as the group aggregate functions (for example, SUM, COUNT, AVG, MIN, and MAX), except window aggregate functions are applied to a window of rows defined by the OVER clause; Benefit: windower queries do not hide the detail. A row for every row; Using the OVER clause you define a set of rows for the function to work with per each underlying row.
- 3. + Example 1 Select custid, orderid, val, SUM(val) OVER(PARTITION BY custid) AS custtotal, SUM(val) OVER() AS grandtotal From Sales.OrderValues; Results
- 4. + Example 1 Description Here’s an example of a query against the Sales.OrderValues view returning for each order the customer ID, order ID, and order value; using window functions, the query also returns the grand total of all values and the customer total. Two differences: the grand total is of course the same for all rows (we are using over with empty parentheses) . The customer total is the same for all rows with the same customer ID
- 5. + Framing Window aggregate functions support another filtering option called framing. Idea: you define ordering within the partition by using a window order clause, and then based on that order, you can confine a frame of rows between two delimiters. You define the delimiters by using a window frame clause. Requirement: a window order clause to be present because a set has no order, and without order, limiting rows between two delimiters would have no meaning
- 6. + Framing You have to define frame units (rows or range) and the delimiters. With the ROWS window frame unit, you can indicate the delimiters as one of three options: UNBOUNDED PRECEDING or FOLLOWING, meaning the beginning or end of the partition, respectively CURRENT ROW, obviously representing the current row ROWS PRECEDING or FOLLOWING, meaning n rows before or after the current, respectively
- 7. + Example 2 Select custid, orderid, orderdate, val, SUM(val) OVER(PARTITION BY custid ORDER BY orderdate, orderid ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) AS runningtotal From Sales.OrderValues; Results
- 8. + Example 2 Description Observe how the values keep accumulating from the beginning of the customer partition until the current row; Because window functions are supposed to operate on the underlying query’s result set, they are allowed only in the SELECT and ORDER BY clauses; If you need to refer to the result of a window function in any clause that is evaluated before the SELECT clause, you need to use a table expression.