The WINDOW clause in SQL is used in conjunction with aggregate functions, particularly in the context of window functions. Window functions allow you to perform calculations across a specified range of rows that are related to the current row, without collapsing the result set into a single output row like regular aggregate functions do.
Here's how to use the WINDOW clause and aggregate functions:
1. Basic Syntax:
The general syntax for using the WINDOW clause is as follows:
SELECT
column1,
aggregate_function(column2) OVER (WINDOW window_name AS [PARTITION BY partition_column ORDER BY order_column])
FROM
table_name
Alternatively, you can define multiple windows for different calculations:
SELECT
column1,
aggregate_function(column2) OVER window_name1,
another_aggregate_function(column3) OVER window_name2
FROM
table_name
WINDOW
window_name1 AS (PARTITION BY partition_column ORDER BY order_column),
window_name2 AS (PARTITION BY another_partition_column ORDER BY another_order_column)
2. Example with SUM():
Suppose you have a table named sales with columns region, salesperson, and amount, and you want to calculate the cumulative sales per region:
SELECT
region,
salesperson,
amount,
SUM(amount) OVER (WINDOW w AS (PARTITION BY region ORDER BY salesperson)) AS cumulative_sales
FROM
sales
WINDOW
w AS (PARTITION BY region ORDER BY salesperson);
3. Example with Multiple Aggregates:
You can also define multiple windows in a single query. For example, if you want both cumulative sales and the average sales amount in different windows:
SELECT
region,
salesperson,
amount,
SUM(amount) OVER w_cumulative AS cumulative_sales,
AVG(amount) OVER w_average AS average_sales
FROM
sales
WINDOW
w_cumulative AS (PARTITION BY region ORDER BY salesperson),
w_average AS (PARTITION BY region);
4. Notes:
- The PARTITION BY clause is used to divide the result set into partitions to which the window function is applied.
- The ORDER BY clause defines the order of the rows within each partition.
- You can also use the ROWS or RANGE specification to define a specific subset of rows within the partition, if needed.
Using window functions with the WINDOW clause provides a powerful means of performing calculations that can give you insights into your data without losing individual row details.