Window Functions

A window function lets you perform calculations on a selection—or “window”—of rows that are related to the current row. Unlike a regular aggregate function, such as avg(), sum(), or count(), the row output of a window function isn’t grouped into a single row.

A window function takes this syntax.

Name	Description
window function	There are two types of window functions. An aggregate function performed over a specified group of rows related to the current row. SQL for CRM Analytics supports these aggregate functions. `avg()` `sum()` `min()` `max()` `count()` `percentile_disc()` `percentile_cont()` A ranking function that returns a rank value for each row in the partition. SQL for CRM Analytics supports these rank functions. `rank()` `dense_rank()` `cume_dist()` `row_number()`
projection expression	The expression that serves as the input to the window function.
`PARTITION BY`	Optional. Splits query results into smaller partitions based on the `reset_group`. When provided, the window function resets after it runs on each part. When you don’t include `PARTITION BY`, all rows are treated as part of a single partition and the function doesn’t reset.
reset_group	One or more columns that reset the windowing aggregation function when their value (or values) change.
`ORDER BY`	Optional. Provides a sorting order for rows in each partition. If ordering rows in a partition isn’t relevant to your query, then don’t include this clause. `ORDER BY` within a window function isn’t the sorting order for your query’s results. To specify the order of query results, include the `ORDER BY` clause at the end of your query.
order_clause	The expressions by which to order the results within a window function partition.
frame_clause	Specifies a subset of rows within the current partition that make up the window “frame.” The frame has a start and end value, such as `ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING`. If no end value is specified, the frame’s end value defaults to the current row. For example, for `ROWS UNBOUNDED PRECEDING`, the frame starts at the beginning of the partition and ends at the current row. See the Example Frame Clause Values table for more examples.
label	The output column name.

Frame Clause Value	Description
`ROWS UNBOUNDED PRECEDING`	From the beginning of the partition to the current row.
`ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW`	From the beginning of the partition to the current row.
`ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING`	From the current row to the last row in the partition.
`ROWS BETWEEN UNBOUNDED PRECEDING TO {number} PRECEDING`	From the beginning of the partition to number rows before the current row.
`ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING`	From the beginning of the partition to the end of the partition. Aggregates the full partition.
`ROWS BETWEEN CURRENT ROW AND {number} FOLLOWING`	From the current row to number rows after the current row in the partition.

A ranking function returns a ranking value for each row in a partition.

Function Name	Description
`RANK()`	Returns a ranking value for each row within a partition. If values are tied, `RANK()` assigns them the same ranking value and skips over the next ranking. For example, if 2 items are ranked at 2, the next-ranked value is 4.
`DENSE_RANK()`	Returns a ranking value for each row within a partition. Unlike `RANK()`, `DENSE_RANK()` doesn’t skip over a rank value if multiple entries are tied. If 2 items are ranked at 2, the next-ranked value is 3.
`CUME_DIST()`	Returns the cumulative distribution between entries in a reset group.
`ROW_NUMBER()`	Returns the sequential number of a row in a partition, starting at 1.

Windowing in SQL for CRM Analytics is available for grouped queries only. This example looks at flights grouped by origin. The query has 3 window functions that demonstrate variations on the 3 parts of a windowing function: partitioning, ordering, and framing.

origin	sum1	sum2	sum3	sum4
PHX	1500	1500	1500	5430
SFO	1950	3450	1950	5430
OAK	1980	5430	1980	5430

This query first executes the WHERE and GROUP BY clauses, so that each expression operates on flights whose destination is LAX. Each group has a column that contains the sum of all of its respective flights’ prices.

The WHERE, GROUP, and HAVING clauses execute before windowing functions.

The subset of data with which we’re concerned looks like this.

Origin	Price
PHX	400
PHX	500
PHX	600
SFO	550
SFO	650
SFO	750
OAK	560
OAK	660
OAK	760

We’ve included sum(price) as sum1 in this example for demonstration purposes only. The window functions return the same results regardless of whether sum(price) is included in the SELECT statement. sum(price) is a regular aggregate function. The expressions that follow it use sum() as a window function.

Here’s the output for the first part of the query calculation.

Origin	sum1
PHX	1500
SFO	1950
OAK	1980

Let’s break down the windowing functions line by line.

The first windowing function is: sum(sum(price)) OVER (ORDER BY sum(price) ROWS UNBOUNDED PRECEDING) as sum2. The window function sum() takes the argument sum(price) as the input for the window function sum2. The clause after OVER specifies the rows and the order in which the function operates on them. Since there’s no partitioning clause, all of the rows belong to the same partition.

The rows are sorted in ascending order of sum(price). ROWS UNBOUNDED PRECEDING says apply this operation—sum(sum(price))—from the beginning of the partition to the current row. Because the query is already grouped by origin, there’s only one value corresponding to each origin. For PHX, that means taking the sum of 1500. For the next row, SFO, the function calculates the sum of sum1 in addition to the unbounded preceding ones—the sum1 value of PHX, 1500 + 1950. For the last group, OAK, the function sums up the OAK total in addition to the two preceding rows, 1500 + 1950 + 1980. This window function computes a running sum of sum(price), ordered by sum(price).

The second windowing function is: sum(sum(price)) OVER (PARTITION BY origin ORDER BY sum(price) ROWS UNBOUNDED PRECEDING). Partitioning by origin returns the groupings that the query established with the initial GROUP BY clause. By including the clause ROWS UNBOUNDED PRECEDING, we take the sum of all of the preceding rows within a particular origin. Since each origin’s prices are already summed up, taking the sum is just each summed value. The results are the same as the results for sum1.

The third windowing function is: sum(sum(price)) OVER (ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING) as sum4. Since partitioning isn’t specified, all rows are part of the same partition. ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING starts the frame at the first row of the partition and continues to the last row. The function aggregates the full partition, which in this example is all of the rows.

This example demonstrates the differences between the RANK() and DENSE_RANK() functions.

origin	s	rnk	denserank
LAX	5400	8	1
HI	3300	6	2
SJC	3300	6	2
ASE	2700	5	3
ORD	2400	4	4
OAK	1980	3	5
SFO	1950	2	6
PHX	1500	1	7

The RANK() function assigns the sum of the prices for all flights from LAX the highest rank. Because the DENSE_RANK() function specifies to order the rows descending, it assigns LAX the rank 1.

Since HI and SJC’s sums are the same value, 3300, instead of assigning them values of 7 and 6, RANK() skips over 7. It assigns them both the rank 6. By contrast, DENSE_RANK() assigns consecutive ranking values. It assigns HI and SJC the value 2. HI and SJC have the next highest descending value after LAX, which has the ranking value 1. The next value DENSE_RANK() assigns to ASE is 3.