Each Merchant's Share of Its Category

Percent-of-parent. Challenge 276 asked for each category's share of the WHOLE ledger and a scalar subquery supplied the denominator. This asks for each merchant's share of ITS OWN CATEGORY, and the denominator is now different on every row — one grand total will not do.

SUM(x) OVER (PARTITION BY category) is the tool: it computes the category's total and keeps the merchant row, instead of collapsing it the way a GROUP BY would. Aggregate to one row per merchant in a CTE first, then window over that.

Return category, name, merchant_spend (rounded to 2 decimals), category_spend (the partition total, rounded to 2 decimals) and pct_of_category (100.0 × merchant ÷ category, rounded to 2 decimals). All 25 merchants. Order by category ascending, then pct_of_category descending, then name ascending.

Round at the end, not inside the window — rounding the inputs and then dividing gives a slightly different number.