Lesson 17 — Row numbering

ROW_NUMBER: number rows within groups

A window function computes a value for each row while looking at other rows, without collapsing them like GROUP BY. ROW_NUMBER() OVER (PARTITION BY x ORDER BY y) numbers rows 1, 2, 3… inside each group x, in order y. PARTITION BY is the "per group" part.

ROW_NUMBER() OVER (PARTITION BY dept ORDER BY salary DESC)

Why it matters: "the top earner per department" or "each customer's first order" become a simple filter on rn = 1.