Oracle Window Functions Explained

In Oracle databases, window functions are special functions used for performing operations such as aggregation, sorting, and ranking. Window functions do not aggregate rows, but instead calculate values on each row in the query result. Using window functions allows for functionality similar to subqueries in a query result set, but with higher efficiency.

The usage method of window functions is as follows:

1. Use window functions in the SELECT clause.
2. Use the OVER clause to specify the partition and ordering of window functions.

For example, here is a sample using window functions to calculate the sales ranking for each department:

SELECT 
    department_id,
    employee_id,
    sales_amount,
    RANK() OVER (PARTITION BY department_id ORDER BY sales_amount DESC) AS rank
FROM 
    sales_table;

In the example above, the RANK() function is a window function that calculates the sales rank for each department. The PARTITION BY clause specifies partitioning for each department, and the ORDER BY clause specifies sorting by sales amount in descending order.

By using window functions, it is easy to achieve ranking operations similar to the above and avoid complex subquery techniques. Oracle database supports various window functions such as RANK(), DENSE_RANK(), ROW_NUMBER(), etc., allowing you to choose the appropriate window function based on specific requirements to implement the desired functionality.