GROUP BY vs a window function?

Definition:

GROUP BY collapses source rows into one row per group, while window functions calculate across related rows without removing the individual rows. A window is defined by partitions, ordering and sometimes a frame. This makes windows useful when a report needs both detail and comparisons such as department rank.

Key points:

• GROUP BY turns each group into a summary row.

• A window function keeps the original rows and adds a calculation.

• Use PARTITION BY for separate groups and ORDER BY for calculation order.

• The ORDER BY inside OVER controls the window calculation, not necessarily the final display order.

Interview point:

• Say whether the result must preserve each original row; that often decides between grouping and a window.