SQL Explained: When to Use Window Functions Instead of GROUP BY

Both GROUP BY and window functions are SQL tools for summarizing data, but they behave very differently. GROUP BY collapses multiple rows into a single summary row per group, losing individual record details in the process. Window functions, by contrast, retain every original row while adding a new calculated column alongside each one. Functions like AVG() with OVER/PARTITION BY, RANK(), and LAG() allow comparisons such as class averages, rankings, and month-over-month changes without reducing the row count. Choosing between the two comes down to whether you need a condensed summary or want to preserve full row-level detail with added context.