Window functions in SQL are a cool feature that let you do calculations across a set of rows related to the current row. It's like you're looking at a "window" of rows in your table and doing things like sums, counts, or averages, but without messing up the results for each row. You can still see all the rows but do calculations at the same time. A regular SQL function, like SUM() or AVG(), usually works on a group of rows and returns just one result, but window functions let you keep all rows and still get those calculations for each one

For example, say you have a table of sales and you want to see the total sales so far for each day. With window functions, you can do that without having to collapse everything into a single row. You can write something like:

SELECT date, amount, SUM(amount) OVER (ORDER BY date) AS running_total
FROM sales;

Here, the window function SUM(amount) is being applied to the rows in the sales table, and OVER (ORDER BY date) defines how the window moves. It calculates a running total of the sales amount for each date but still shows each date and amount as separate rows. So, the key part is that you're using the OVER clause to define how the "window" works, and it doesn't remove rows like a GROUP BY would

You can also do things like ranking rows. There's a ROW_NUMBER() function that gives each row a number based on its position. Or RANK() and DENSE_RANK() which help when you want to rank rows but deal with ties differently. For example, if two rows have the same value, they might get the same rank number but the next number could be skipped depending on which rank function you use

So, window functions are super useful when you need to do calculations but want to keep the details of individual rows. They let you perform tasks like finding moving averages, calculating running totals, ranking data, or finding the difference between the current and previous row.

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