I've been working with SQL off an on for 20 years and never heard the term window functions before. TIL.
this post was submitted on 15 Jun 2026
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Think of them as operating on the whole column at once.
For example LAG would be like selecting a whole column in excel and copy pasting it one row down
Think of window functions as a two-layer operation:
Layer 1: Produce all the rows (FROM, WHERE, GROUP BY, HAVING)
Layer 2: For each row, peek at its "neighborhood" (partition) and compute something
The result of Layer 2 is just another column added to each row. Nothing collapses, nothing gets removed. You just get extra computed values based on context and that context is what PARTITION BY, ORDER BY, and the frame clause define.
Thinking windows functions in two layer actually helped, Thank you kind stranger! :)
The main thing is that group by gives you a smaller number of rows than the input, collapsing the data from individual rows into the group rows in the output, while a window function does not change the number of rows - it just allows you to use data from other rows for calculations in each output row