What is the difference between a window function and the GROUP BY operator?

The window function operates on a set of rows related to the current row (window) and returns a single value for each row. The GROUP BY operator groups identical values into one set and returns a single aggregated value for each group.

-- Example of a window function: numbering rows within a group
SELECT
    order_id,
    order_date,
    ROW_NUMBER() OVER (PARTITION BY order_date ORDER BY order_id) as row_num_in_day
FROM orders;

-- Example of GROUP BY: counting the number of orders by date
SELECT
    order_date,
    COUNT(order_id) as orders_count
FROM orders
GROUP BY order_date;

Feature	Window Function	GROUP BY
Number of rows in result	Preserves all rows of the original set	Reduces the number of rows to the number of groups
Application	Calculations "per group" without collapsing it	Aggregation and collapsing of groups
Syntax	Uses OVER()	Uses GROUP BY
Access to rows	Access to individual rows within windows	Access only to the aggregated value of the group