SQL for Sellers With Degraded Fulfillment | Dataford Interview Questions

Whatnot Operations needs to identify top-performing sellers whose shipping fulfillment has worsened recently. Write a PostgreSQL query comparing the last 30 days with the preceding 30 days, using August 29, 2026 as the reporting date.

Requirements

Calculate each seller's on-time shipping fulfillment rate, where an order is fulfilled when shipped_at is on or before promised_ship_date.
Define top-performing sellers as those with at least 2 preceding-period orders and a preceding fulfillment rate of at least 90%.
Return sellers with at least 2 recent orders whose fulfillment rate declined by at least 15 percentage points.
Return both period rates, order counts, and the decline, ordered by decline descending.

Schema

sellers

Column	Type	Description
seller_idPK	INT	Unique seller identifier
seller_name	VARCHAR(100)	Seller display name

orders

Column	Type	Description
order_idPK	INT	Unique order identifier
seller_id	INT	Seller who created the order
order_date	DATE	Date the order was placed
promised_ship_date	DATE	Required ship-by date

shipments

Column	Type	Description
shipment_idPK	INT	Unique shipment identifier
order_id	INT	Order associated with the shipment
shipped_at	DATE	Actual shipment date

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