Find Customers Inactive for 30 Days | Dataford Interview Questions

A retail platform wants to identify customers who need re-engagement. Write a SQL query to find customers who have not made a purchase during the 30-day period ending on September 26, 2026.

Output

Return one row per inactive customer.
Include customer_id and customer_name.
Include customers with no purchase history and customers whose purchases are all older than the 30-day period.
Purchases with a NULL date do not count as purchases within the period.
Sort by customer_id in ascending order.

Schema

customers

Column	Type	Description
customer_idPK	INT	Unique customer identifier
customer_name	VARCHAR(100)	Customer display name
city	VARCHAR(80)	Customer city

purchases

Column	Type	Description
purchase_idPK	INT	Unique purchase identifier
customer_id	INT	Customer who made the purchase
purchase_date	DATE	Date on which the purchase was made
amount	NUMERIC(10,2)	Purchase amount

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