ROW_NUMBER

The previous chapter introduced window functions and how they compute values across rows without collapsing the result. But aggregate window functions like SUM and COUNT produce computed values. They don't tell you where a row sits within its group.

ROW_NUMBER fills that gap. It assigns a unique sequential integer to each row inside a partition, starting at 1, based on the order you specify. That simple numbering unlocks three of the most common SQL interview patterns: pagination, deduplication, and top-N-per-group.