Break the problem into two parts: first identify categories that have books by at least 3 distinct authors, then count purchases only from those categories and rank customers by purchase count. Use a subquery or CTE to filter categories, then join with Purchases and aggregate by CustomerId, finally order and limit to top 3.
Pro tip: Clarify whether 'top 3 customers' should include ties or if a strict limit of 3 is required, and mention that you'd handle ties by either using RANK/DENSE_RANK or by discussing with the interviewer. Also, consider if the same book purchased multiple times should count multiple times—usually it should, but confirm.
Write a subquery to find categories where the number of distinct authors (from Books) is at least 3. Join Books with Purchases on BookId to get authors per category, then group by Category and filter with HAVING COUNT(DISTINCT Author) >= 3.
Use the result from step 1 to filter the Purchases table, keeping only rows where Category is in the qualifying set. This can be done with an IN clause or a join.
Group the filtered Purchases by CustomerId and count the number of purchases (e.g., COUNT(*) or COUNT(BookId)). This gives the total qualifying purchases per customer.
Order the aggregated results by purchase count descending and limit to 3. If ties are a concern, use a window function like RANK() or DENSE_RANK() and then filter for rank <= 3.
Mention potential edge cases: customers with zero qualifying purchases, ties at the boundary, and whether to include customers with no purchases. Also, consider performance implications and indexing.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.