Top 3 Merchants Per Category by Spend

Top-N per group — the window question every analyst screen keeps. For each merchant category, rank merchants by their total card spend and keep the top three. Aggregate first (one row per merchant), then rank inside a CTE with ROW_NUMBER() OVER (PARTITION BY category ORDER BY total_spend DESC, merchant_id), then filter rank_in_category <= 3 outside it — a window function cannot sit in a WHERE clause.

Return category, name (merchant name), total_spend (SUM of amount, rounded to 2 decimals), and rank_in_category. Categories with fewer than three merchants return what they have. Order by category ascending, then rank_in_category ascending.