I went straight for a join across all four tables and then tried to filter in the WHERE clause, which almost immediately broke things because you end up excluding rows you need for the other item.
Start by clarifying the schema and the definition of 'purchased both' (e.g., within the same order or across any orders). Then write a query that joins customers to orders to order items to items, filters for Kindle and Alexa, and counts distinct customers who have both products using a self-join or conditional aggregation.
Pro tip: Mention that you would verify the query's correctness with edge cases (e.g., customers who bought multiple Kindles, or only one of the products) and discuss performance considerations like indexing on item names and customer IDs.
Ask whether 'purchased both' means in the same order or across any orders, and confirm the table relationships (e.g., customers->orders->order_items->items).
Filter the items table to get the item IDs for 'Kindle' and 'Alexa', or use a subquery/CTE to isolate these products.
Join customers, orders, order_items, and items to link customers to the products they purchased, ensuring you capture all relevant orders.
Use conditional aggregation (e.g., COUNT(DISTINCT CASE WHEN ...)) or a self-join to find customers who have at least one Kindle and at least one Alexa purchase, then count them.
Test with edge cases and consider indexing or query performance improvements, such as filtering early or using EXISTS clauses.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.