Window Functions vs Aggregate Functions in SQL: A Beginner-Friendly Guide
If you have written SQL for more than a week, you have used SUM() , AVG() or COUNT() . They are the bread and butter of reporting. Then one day someone says, "Just use a window function for that" , and suddenly you are staring at a query with OVER (PARTITION BY ...) in it and wondering what you missed. This article is for you if you are just starting with window functions , or if you already know…
Window functions and aggregate functions are both used in SQL for aggregating data, but they differ in how they handle the rows of data. Aggregate functions such as SUM(), AVG(), and COUNT() collapse many rows into a single row by calculating a single value for each group of rows. This is achieved using the GROUP BY clause. In contrast, window functions perform the same calculations across many rows but keep every original row in the result set.
This is indicated by the use of the OVER (PARTITION BY ...) clause. The key difference is granularity – after using aggregate functions, each row represents a group (e.g., a product), while after using window functions, each row still represents an individual record (e.g., a sale). This allows window functions to provide more detailed information while still performing calculations over sets of rows.
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