Window functions are one of SQL's most powerful features, but many developers avoid them because the syntax looks unfamiliar. This guide explains when to use them and how to write them correctly. What is a window function? A window function performs a calculation across a set of rows related to the current row — without collapsing the rows into a single output row (which is what GROUP BY does). The key difference: -- GROUP BY: one row per group (collapses rows) SELECT department , AVG ( salary ) as avg_salary FROM employees GROUP BY department ; -- Window function: keeps all rows, adds AVG per department SELECT name , department , salary , AVG ( salary ) OVER ( PARTITION BY department ) as dept_avg_salary FROM employees ; Enter fullscreen mode Exit fullscreen mode With the window function, you can see each employee and the department average on the same row. This is impossible with GROUP BY alone.…