SQL for Top Buyers | Dataford Interview Questions

Etsy wants to identify its most active buyers using recent purchase activity. Write a SQL query to find the top 10% of active buyers based on purchase frequency during the previous 90 days, including their spending and most frequently purchased product category.

Output

Return one row per buyer in the top 10% of active buyers during the 90-day period ending on September 15, 2026.
Include buyer_id, purchase_frequency, total_spend, and primary_category.
Define primary_category as the category with the most purchases by that buyer. Break category ties alphabetically, placing a NULL category last.
Rank buyers by purchase frequency descending, total spend descending, and buyer ID ascending. Return the top decile, rounded up to at least one buyer.
Sort the final result using the same buyer-ranking order.

Schema

buyers

Column	Type	Description
buyer_idPK	INT	Unique Etsy buyer identifier
buyer_name	VARCHAR(100)	Buyer display name
email	VARCHAR(255)	Buyer email address

purchases

Column	Type	Description
purchase_idPK	INT	Unique purchase identifier
buyer_id	INT	Buyer who made the purchase
product_id	INT	Purchased listing identifier
purchased_at	TIMESTAMP	Timestamp when the purchase was made
amount	DECIMAL(10,2)	Purchase amount in dollars

products

Column	Type	Description
product_idPK	INT	Unique listing identifier
product_name	VARCHAR(150)	Etsy listing name
category	VARCHAR(80)	Product category

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